Tower

Live map — https://apps.fuzzyworld.net/tower/ · Source

Companion piece to a novel set inside Tower — a fictional smart cylinder orbiting Earth, spun for 1 g, with an inner surface the size of Wisconsin, lit by 54 TW of sunlight piped down its axis and scattered by mirrors that make its days and nights. Its wall is a kilometre of sealed hull: no windows along all 540 km, and no view of the Earth it orbits. Both end caps are windows, though — 100 km of glass each, and the only places in the world with a horizon that is not Tower. Nobody aboard can see the sun either: the 244 km collector sits between Tower and it, 79 times its diameter, so the sun cap looks out at a black disc and the stars around it — black except at its edge, where the structure holding it flat catches the sun side-on and traces the whole 244 km in a thin sun-coloured ring.

Explore Tower from outside, pick a spot on the inner surface, and stand there — with the ground curving up on both sides and the far side of the world hanging 100 km overhead, clearer than the ground at your feet. Turn its sun up to Earth clear-sky noon, down to permanent dusk, or shut the beam off entirely and watch the sky come out: through the far cap the stars wheel once every 7 min 29 s about a single motionless star on the axis. Nothing inside Tower rotates, because you and the ground and the air all turn together — so that window is the only place the spin is visible at all.

Tower flies all 78 billion mirrors of its own sunlight, individually, because nothing passive would stay in the beam past mid-afternoon. It is a person.

Status

All three views are built — map, exterior and interior — over a world that generates itself live, in the browser, from a file of authored features.

Built: the unrolled surface map, Tower from outside, standing inside it and flying over it, a detail card for every place, Tower's published weather, a sun that crosses the world each day on Pisa's clock and a moon by night with Earth's phases, two railways, a scenic tour that flies the world's paragliders, aircraft, airships, trains and spacecraft inside and out, permalinks, and the sightline probe.

The app is for looking; the world is changed in the repository. The author writes alongside Tower rather than in it, and changes the world by talking it through with Claude, who edits content/world.json. That file is the one authoritative copy — there is no editor in the browser, no local draft, and nothing to import or export (decisions.md D-072).

In progress: world-generation.md. Phases 1 to 5 are built — the terrain is no longer baked into images by a build step; every mountain, lake, sea, river, desert and forest is a feature the browser turns into ground on demand, in web workers, from a metre underfoot to the whole world. Mountain ranges are summits joined by ridges and cut by water into valleys; lakes, seas and rivers are water with waves, flow and waterfalls; Tower publishes a weather plan, and ships sail the ring seas by it. The Dry Reach has mesas, dune fields, dry washes and salt pans, and Greatwood is three billion individual trees with meadows through it and a wood of 100 m giants near the Capital. Phases 7–8 add settlements and roads.

• design.md — canon, physics and application architecture
• world-generation.md — how the world is generated, and the plan
• decisions.md — choices made and why

Both are series-bible material: coherence outranks spectacle, and the prose is downstream of them.

On a phone the sidebar is a drawer rather than a column: below 720 px the world gets the whole screen and the panel slides in over it, one tap away and one tap gone. Picking a place closes it, because in every view selecting a place is how you travel there — see decisions.md D-043.

Screenshots

npm run screenshots drives the real application in a headless browser — clicking the view buttons, picking places out of the list, dragging to orbit and to look up — and writes docs/screenshots/. Taking them through the UI is the point: a shot taken this way fails when the UI fails, so a black canvas ends up in the picture instead of hiding behind a passing unit test.

A scene the tour photographs is held at one moment with the water's clock stopped too, so the sea in a shot is the same sea every run and a divergence means a real change. --only <ids> retakes a few shots and leaves the sheet alone.

The whole tour takes about three minutes, and waits on the world being generated rather than on fixed delays: a shot is taken once the terrain has compiled, the map is drawn, and every tile of ground the view wants has arrived. It is kept light on the machine: the world is compiled once for the whole tour and every shot starts from that cache, and each page runs two terrain workers rather than a full pool — four pages at once each compiling the world with six workers was a laptop running hot. It once took six and a half minutes, nearly all of them spent rendering Tower's haze integral on the CPU — the default headless browser has no GPU. It now asks for the full Chrome build, which reaches the real one, and photographs four shots at a time, since a shot is mostly spent waiting. The cost is that the committed images are tied to the GPU that drew them: another machine will show a large one-off divergence rather than a regression.

Looking before committing

overview.png is a contact sheet of every shot. It exists to be looked at, once, before committing — one image that answers "is the application still there?". Regenerate, glance at it, then commit.

--check puts a number on the same question. Every shot is compared against the committed one and the run fails if any of them moved too far:

npm run screenshots -- --check              # fail past 3% divergence on any shot
npm run screenshots -- --threshold 8        # report only, looser bar
npm run screenshots -- --url http://localhost:5173   # use a dev server already up

The threshold is measured at both ends, not guessed. The 2D shots come out byte-identical between runs — nothing in Tower is stochastic, the exterior spin is switched off, and every shot gets a fresh browser context so no camera leaks into the next. The WebGL shots do not, because the tour draws on the real GPU and Metal does not schedule floating-point work identically twice; they drift by 0.1% to 0.9%. Against that, hiding both 3D canvases scored 7% to 31%, and a view going entirely black scores 12% to 39%.

3% is three times the worst noise and less than half the mildest real failure. It is a smoke alarm, not a verdict: when it fires, go and look at the sheet. Note that a view which never worked scores 0% when it breaks further — this catches regressions, not things that were already broken.

Photographs

A view worth keeping — things lined up, at a particular time — is saved as numbers and rendered again whenever it is wanted, so an illustration can always be remade from the latest world and the latest renderer (D-099).

1. npm run dev, open the exterior or the interior, and frame the shot: the camera, the day and time, the Light, the hull.
2. Press P, or Save this shot under View, and give it a name. The shot goes into content/photos.json. Saving under a name already in the list re-frames that shot. The button and the key exist only on a dev server, and it takes a shot only from the machine it runs on; the published site has neither.
3. npm run photos renders every shot to photos/<name>.png.

To go back to a shot, pick it from Saved shots, under the button: the app opens its view, puts the camera where the shot has it, and stops the clock at its moment (and, outside, the spin), and carries on from there. Adjust it and save under the same name — the name is filled in — to re-frame it. The list is read from the file each time it is opened, so a shot typed by hand is in it too.

npm run photos                              # every shot in content/photos.json
npm run photos -- --only far-cap            # just the shots whose names start so
npm run photos -- --long-edge 5760          # bigger; the default is 3840 (4K)
npm run photos -- --url http://localhost:5173   # use a dev server already up

A render is the view alone — no panel, no readouts, no gauge, no labels — at the shape of the window the shot was framed in, 3840 px along its long edge. The file name never changes, so rendering again replaces the picture with the newest version of itself. photos/ is not committed: the list is what is kept, and the pictures are made from it.

An entry is plain numbers, and is meant to be edited by hand as well:

A render holds still: the hull does not turn, the sky through the far cap does not wheel, and the water is held at one moment, so the same entry gives the same file every time. A shot cannot be saved while a scenic tour is running — its flyers belong to the tour — so stop the tour first.

The list holds the two caps as seen from 300 m over the Capital at 08:45, and tower-from-the-far-end, the blog illustration of 2026-08-19 worked back from its JPEG: the camera and the field of view fitted to where the far cap, the collector and the sun fall in it, the day from the stars and the hour from the lit rod. How far the hull had turned could only be told to a sixth of a turn, so the rock is not lump for lump the same.

Development

npm install
npm run dev      # http://localhost:5173, plus a LAN URL and a QR code for it
npm test         # 279 tests; canon figures are pinned to design.md
npm run build
npm run screenshots  # drive the app and rephotograph docs/screenshots/
npm run screenshots -- --only tour-grand   # just those shots, for working on one view
npm run photos       # render the saved shots in content/photos.json to photos/

npm run dev binds to every interface (vite --host) and prints a scannable QR code of the network URL, so the touch interactions can be tried on a phone on the same Wi-Fi without typing an address. The server is reachable by anyone on that network while it runs; drop the --host if that is not wanted.

Vite + TypeScript, deployed to GitLab Pages by .gitlab-ci.yml on every push to the default branch. Three.js is loaded only when a 3D view is opened, so a reader who stays on the map never downloads it. There is no terrain build step: the world is generated in the browser from world.json, which the build makes from content/world.json (world-generation.md §5).

Every branch is on Pages too. The default branch is at the site root and every other branch at /tower/<branch>/ — forest at https://apps.fuzzyworld.net/tower/forest/ — so a branch can be looked at without a dev server. When the site holds more than one branch, a branch picker in the panel header switches between them, keeping the view and place in the URL. tools/build-pages.sh builds them all from the default branch's pipeline, and a push to any other branch triggers that pipeline, so a preview is a few minutes behind its branch. A branch that fails to build is left out rather than taking the site down. All of them share one origin, so they share browser storage: the remembered view, camera and label setting follow you between branches.

Assets are referenced relatively, so the same build works whether Pages serves it from https://apps.fuzzyworld.net/tower/ or from a generated unique domain at a root. Set a BASE CI variable to force an absolute base if that is ever needed.

When the interior view looks wrong

The interior keeps a flight recorder — the last quarter-minute of camera, draw calls and frame colour — because the rendering faults worth catching there have all been intermittent (decisions.md D-057, D-058). While the fault is still on screen, open the console and call:

towerDump()   // logs the whole state and copies it to the clipboard

The first line is the answer most of the time. It draws each surface alone and measures how colourful it is:

[tower] alone in frame — ground 31.32, sunCap 19, farCap 31.3, bore 19, beam 19  (19 = nothing drawn)

19 is the clear colour, so it means that surface put nothing in the middle of the view — expected for anything currently off screen. The ground reads about 30 when it is carrying land. A ground near 19 while you are looking at the ground is the diagnosis: the ground's tiles are missing or its shader is failing. The dump's terrain entry says which: the world generation the view is drawing, how many tiles are on screen, whether every tile it wants has arrived, and how busy the worker pool is. The rest carries the camera and the per-frame trace, with the tile count in every frame, so the heading the view changed at can be read off afterwards. window.tower is the view itself, and window.towerTerrain the terrain service, for poking at directly.

This is on in the built site as well as in dev, deliberately: the fault has only ever turned up in ordinary use, and it costs two properties on window that do nothing until they are called.

Geography

content/world.json is the world: every feature — mountains, lakes, seas, rivers, desert, forest, settlements and plain markers — with its shape in canon coordinates (θ, z) and its parameters beside it, plus the author's notes. The browser generates the terrain from it live (world-generation.md, decisions.md D-062); there is no build step and there are no terrain images. It is the only copy: what the app loads is world.json, made from it by vite.config.ts — served by the dev server, written into the build — with every note removed and any feature marked "public": false left out. It is never committed, so it cannot drift from the file it comes from. Save content/world.json with npm run dev running and the open page reloads with the change.

Generation is deterministic. The seeds are in the file, so every reader sees the same world the author does, to the tree.

The compiled world is kept in the browser's IndexedDB, keyed by the world and the build that compiled it, so a returning visit to an unchanged world skips compiling.

When the world loads, and after any edit that changes the ground, it is compiled in a worker — rivers given their long profiles, lake levels found, summits measured. If that takes longer than 400 ms a dialog says what Tower is making, one step at a time. A rename, a blurb or a note changes nothing on the ground and regenerates nothing.

The world was ported from v1's content/geography.json by tools/port-world.mjs, kept for reading; there is no backwards compatibility with v1 files.

Reading

The app only reads the world. Select a place — in the list, on the map, or in the interior — to see its detail card: its name, its blurb, where it is, and what the world made of it — a range's named summits, the sea running on a lake or sea now by the clock, how many trees a forest holds and how far apart, what a desert's landforms came to. Shift-click two points on the map to measure a sightline (Esc clears it), and F fits the map. Show labels in the View section — or L, in any view — hides or shows every label at once: the map's place names and captions, and the captions over the exterior and interior. The choice is remembered across reloads.

Changes to the world — a new place, a moved summit, a storm in the weather plan — are made in content/world.json, not in the browser (D-072).

Any selection updates the URL, so a link is always shareable:

Views

Map is the unrolled inner surface. Exterior is Tower from outside — a kilometre of hull, a window at each cap, the real night sky, the 244 km collector flying ahead and the beam converging into the sun cap. Each piece is captioned.

The stars are Earth's — every naked-eye star in the Yale Bright Star Catalogue, about 9,100 of them to magnitude 6.5, at their true positions, brightnesses and colours. The same sky is seen through the cap windows from inside. Tower's axis points at the sun, so the far cap looks at the point opposite it, and the sky behind Tower turns once a year with the clock's date; from outside, ecliptic north is up. node tools/bright-stars.mjs rebuilds the catalogue file (src/data/bright-stars.json) from CDS.

The hull reads five ways and the Hull switch picks one; it opens on True and remembers whichever you last chose. True is the outer skin: bare structure, and both caps real windows — look in through either and the land is actually there, hazed exactly as the interior view hazes it and receding 540 km down the tube. It is the only line of sight into Tower that is not a diagram. X-ray wraps the terrain onto the outside instead, which is a diagram of what is inside rather than a view of it, and useful precisely because the wall is opaque. Rock and Piping are experiments, not canon. Rock dresses the hull as an asteroid: a cratered, lumpy shell of rock a few kilometres thick laid over the skin, with the windows left clear. Piping dresses it as industrial plant: panelled plating, a band with hoop pipes every ~36 km, bundles of pipes 0.6–2.2 km across running between them, and blocks of plant. Vents open at random, each on its own clock: grilles flare hot and fall back to an ember, and exhaust shoots out and fades. Near each end are thruster pods — what holds Tower's attitude — and every so often one fires a long flame that fans out into space and lights the plating round it. All of it wears a layer of dust, thickest on whatever faces outward. Rock and piping puts the two together: the plant half buried under banks of rock heaped on as shielding, bare plating and plant rooms in the gaps, the bands and pipe runs going under the rock and out again, and the rock dug clear round each thruster pod. The active view is remembered and travels in the permalink, so #view=exterior&place=the-narrows opens where you left off. Drag to orbit, scroll to zoom, click the hull to drop a point. Scenic tour in the exterior's panel flies spacecraft instead: skimming the hull while it turns under them at 700 m/s, docking on it — ships dock anywhere on the shell, and hang from it at 1.02 g — looking in through the far cap, over the collector's rim, down the outside of the beam, and Tower, the collector and the sun in one frame; the spin runs at 1x to 1200x real time, and a point picked off the turning hull still comes back in canon coordinates.

Interior is standing inside. The ground is the real cylinder, built from generated tiles that get finer around you — half a metre between vertices underfoot, half a kilometre on the far side of the world — so it curves up on both sides and closes overhead. Haze is integrated through Tower's rotational air column for every pixel, which reproduces the strangest fact about the place: the land 100 km straight up is 78% clear while the ground 100 km away is 31% — distant geography here is best seen by looking up.

One finger looks and two fingers move (D-059). Dragging with two takes hold of the ground under them and keeps it under them, so the distance covered comes out right by itself and the tilt of your head is the gearing: level, a screen of drag walks you 1.9 km, and tipped 23° down it steps 26 m. On a trackpad the two-finger swipe moves you too, and so does a mouse wheel: a screen's swipe walks about 70 m from standing and some 40 km from a kilometre up, the ground following your fingers. A mouse has only ever one pointer, so shift-drag or the middle button stands in for the second finger. There is no zoom: the field of view is fixed at 60° (D-083). Selecting a place — in the list or by its caption — still walks you there, which is the only way to cross the world in one move: most of it is beyond the haze horizon from any given spot, so travelling is more use than turning.

The forest is trees. Greatwood is about three billion of them, so only the ones you could see as trees exist at any moment: meshes within a couple of hundred metres, then cards turned to face you, then clumps — one card for every thirty metres of wood — out to nearly three kilometres, over a canopy that closes the ground behind them, with the meadows showing through as grass. Which trees stand where is a rule rather than a list, so they are the same trees for everyone and they are still there when you walk back. The Tall Wood is 52 km² of giants near the Capital — 90 to 115 m, about as tall as a tree can be at 1 g — placed once from the world's seed and then left where it fell.

A settlement clears the trees it stands on, so the Capital is a 20 km opening in Greatwood rather than a city in a wood.

The desert is a desert. The Dry Reach rings the world with tiers of rock standing over dune fields, hills, dry washes routed the way water would have routed them, and salt pans in its lowest ground with the shorelines of lakes that never stood stepped around them. None of it is a remnant: Tower had seen deserts on Earth and built one.

The water is water. Lakes, seas and rivers are surfaces at their own levels, with waves sized from each body's wind and fetch, rivers running downstream, and rapids white. A river that cannot slope falls: the Long Water drops 680 m off the range into a gorge, and the falling water drifts antispinward as it goes — the spin, shown by what falls.

The weather is Tower's, and it is published. Waves follow Tower's weather plan: a storm builds the sea over a few hours, whitecaps spread with the wind, and the sea keeps running after the wind drops. Ships sail both ring seas, each on its own lane round the world, riding the waves drawn under it with its mainsail set to the wind. When a gale or storm has been published for their sea in time, they stay in harbour — even if Tower has since called it off, because people act on what they were told — and a squall announced too late catches them at sea. Set the Day under Time, or run the clock, to move through the plan.

Or take the scenic tour. Scenic tour in the view panel hands you from one of the world's flyers to the next, every twelve to eighteen seconds, in an order dealt at random: a paraglider on Snowcrown's flank, a glider circling up in a thermal over the Dry Reach, a helicopter round the great waterfall, a seaplane across The Deep, an airliner round the Girdle — flying antispinward, and so weighing a third of what it would at rest — a giant airship climbing out of the Capital until the world closes overhead, another circling the sun, outside the beam as every aircraft must be, trains on the two railways, and ships in fair weather and in a squall. The camera rides in the craft or chases it, banks with it, and turns to whatever can be seen from there — never at something the haze has eaten. The slow legs are time-lapses and the caption says so. The caption's border fills in clockwise from its top-left corner as the leg runs, and the next leg comes when it closes. N skips ahead; Esc, a drag or a flight key stops it and leaves you where it was.

Or fly. W A S D or the arrow keys move you, E climbs and Q descends, and shift goes five times faster. Speed follows your height above the ground: a brisk walk at standing height, and a sea crossed in seconds from ten kilometres up, with nothing to set in between.

The caps have more than one name. The sun cap is the end the beam comes in at. The far cap is also the star cap, since it is the one that shows the stars, and the night cap, since the night starts there: the sun goes out at it, and the moon sets out from it. More names get added as they come into use.

The far cap is a window — 100 km of it, looking into deep anti-sun space. By day it is simply a disc of blue, because 540 km of air in front of it scatters more light than anything behind it sends; the stars are there at noon and unseeable, which is a fact about eyes rather than about glass. At night they come out. The sun cap is glazed too: it faces the collector, not the sun, so it has nothing to manage the glare of (design.md §1.3.0).

The one part of the far cap that is not glass is a 3.2 km mirror on the axis. A quarter of the beam survives all 540 km and arrives collimated, so it lands as 1.7 MW/m² — 1,200 suns — on a spot a thousandth of the cap's area; the mirror sends it back up the column for a second pass, which lifts the beam's utilisation from 76% to 94%. The rod reaches it at every hour: by day the sun takes part of the beam and the rest runs on past it, so the beam is one line of light from cap to cap with the sun moving along it, and the mirror is a lit disc where the rod ends.

Both caps' windows are framed by mullions in a lattice of identical equilateral triangles, 8.3 km on a side, six meeting at the axis. That is the pitch of the checker the caps once had, so the frame is still the only object in the world with a known size and the only rule anyone has for judging distance, read against sky.

The far end of the world is cold. The far cap's glass is single, and glass does not let heat out as light: it soaks it up and radiates it to space from its outer face, 1–2 TW over the whole cap. So the window is a cold wall, with a cold wind running down it onto the Deepward Sea, and Tower keeps that end dim and lets it freeze: open water at freezing, then pack ice, then fast ice, dusk by day and dark by night — and against the window ice mountains, the Cap Range, 2 to 3 km high: rock Tower raised at the cap's foot for the ice to stand on, since ice alone spreads out before it gets higher than a few hundred metres. The glass frosts too, and Tower clears it one pane per turn of the world, in an order reshuffled every 35.1-hour loop, so close to the window is white with a scatter of clear, starry triangles — from inside and from outside. Tower can do this because it controls how much heat the hull loses where, up to about 5 TW (design.md §1.9.4, §1.9.6; D-090). The sun cap is double-glazed and does none of this: the Sunward Sea is a temperate one.

The farms keep their own days. Nine farms round the world — on the Uplands, in clearings cut out of Greatwood, and before the far end where the railway ends — share 250 km² of farmland — tulips, lavender, sunflowers, rapeseed, poppies, flax, maize, potatoes, tomatoes, wheat and more, most of it in bloom — is lit by Tower as each crop wants: as at noon from sunrise for twelve to eighteen hours, long-day crops on into the night and short-day ones dimmed early, and every one of them given some darkness, which flowering plants need. After sunset the fields blaze in a dark world and go out one crop at a time (design.md §1.9.7; D-095).

Tower has a sun, and it crosses the world once a day. The mirrorlets that fly in the beam gather into one shoal and throw the beam's light out sideways there, which makes a single blazing sun at one point on the beam. It lights at the sun cap at sunrise, moves at a steady pace down the whole length of the world, and reaches the far cap at sunset, where it goes out: that is nightfall. Through the night the beam is banked to a thousandth, and the next morning the sun lights at the sun cap again. So Tower's sun moves from one end of the world to the other, every day.

Tower keeps Pisa's days. Its clock is Pisa's — Italian time, daylight saving included — and its sun comes on at Pisa's sunrise and goes out at Pisa's sunset, so the days lengthen and shorten with Pisa's seasons: about 44 km/h at the equinoxes, when day 0 falls, 61 km/h at midwinter and 35 km/h at midsummer. The city and the date of day 0 are the calendar in content/world.json. (Until D-089 it kept Sydney's.)

Every place has its own noon. Tower aims the swarm so the sun is as bright seen from anywhere as seen from under it, so the ground is lit by how high the sun stands, as it is under Earth's. From The Capital, halfway along, the morning sun is low in the sunward sky, it passes overhead at midday, and it sinks away deepward in the afternoon; it never sets, since 50 km up and 540 km away it is still 5° high. The beam glows from cap to cap at every hour, a little dimmer past the sun, which takes part of it and lets the rest run on to the far cap's mirror (D-079). Inside and out it is the same sun: the exterior looks in through the windows at the one the interior stands under.

By night Tower has a moon. A second, smaller gathering of the swarm lights at the far cap as the sun goes out there, and crosses the world the other way through the night, reaching the sun cap by sunrise. It is as lit as Earth's Moon is that night — full, gibbous, a crescent lit toward the far cap while it waxes and toward the sun cap while it wanes, or nothing at new moon — and it lights the ground by it: a full-moon night is twice as bright as a moonless one, with a path of light across the water toward it. The Time section says where it is and how lit, and the map draws a pale line where it is overhead.

The clock opens at the time in Pisa now, and runs. It starts from Pisa's current date and time and goes forward at whatever speed it was last run at — 900x until you choose another, and 1x keeps it in step with Pisa. Add ?clock=<hours> to the address to hold it at a moment instead, stopped: ?clock=12 is noon on day 0. That is how the screenshot tour gets the same sky every run.

Time, in the panel, works in every view: the Day (with its date in Pisa), the Time of day, Run the clock and its rate, and the Light — how bright Tower runs its day. It says when the sun rises and sets and where it is now, and the map marks it. Dark shuts the 54 TW beam down entirely and is the only setting that is genuinely black — no sun, no airlight, no rod, and nothing left in the world but the sky. Twilight, Temperate and Full set how bright the day is; night comes by itself, with the clock.

Eye height is a gauge up the right-hand side of the view, from 1.7 m to the beam, 48.4 km up, on a log scale: drag it, or click it and use the arrow keys. It is marked with things of known height — an adult, a two-storey house, the Eiffel Tower, Everest, an airliner cruising, a weather balloon, and at the top the beam itself — which light up as you pass them, and the readout by the thumb gives the height, the local gravity, and what it compares with ("≈ the Eiffel Tower", "2.0× the Burj Khalifa"). On a phone that readout fades once the height has been still for two seconds, and comes back as soon as it changes or you touch the gauge. The Where you stand readout in the bottom-left corner folds down to its title with a click, and stays as you left it; on a phone it starts folded.

Nothing inside rotates, because you, the ground and the air all turn together. The exception is the window: the sky out there does not co-rotate, and wheels once every 7 min 29 s about the one motionless star on the axis. It is the one sign of the spin anywhere in the view, and it is only available at night, at the deepward end, looking the one direction that has ever had anything in it but Tower.

What the world file says

Each feature in content/world.json has a kind, a shape — a point, an open line (a river runs source first), an outline, or a band ringing the world — and its kind's parameters beside it. Changing any of them is a conversation and an edit to the file; the ground regenerates on the next load. This is what the parameters mean.

A mountain range has named summits, each held at exactly its elevation, and parameters for how high its unnamed summits stand and how many there are, how rugged it is, how deeply water cuts it, and its snowline. A change regenerates the range — a few seconds for the Girdle, under the status dialog.

A lake or sea has a wind: the direction it blows toward and its speed. That is its usual wind, which blows whenever the weather plan has nothing scheduled. The detail card says what sea is running there now, by the clock — a 10 m/s wind over The Deep makes waves under a metre; the same wind round a ring sea, which never runs out of water to blow over, makes 2 m, and 20 m/s makes 8.6. A sea also has a number of ships.

Weather is Tower's plan: events over a lake or sea, each published at a moment and revisable later with a reason — the storm eased because someone foolish went sailing — and a number of days the plan takes to repeat. The panel's Weather section shows what is blowing now and the next fortnight, as far as it has been published, with any revision listed under the event it changed and the original struck through. None of it regenerates the ground.

A settlement has a size — hamlet, village, town, city or station — and how far it reaches. It is a point with an extent, not a traced outline: Tower lays the streets out inside it. The Capital is 20 km across, centred on the axial midpoint of the world, which is just wide enough to put its spinward edge on The Deep's shore.

A forest has what Tower planted there: stems per hectare, how much of it is conifer, how much of the ground is left open as meadows, the treeline, and whether it is a wood of giants. The detail card says what that comes to — how many trees, over how much ground, how far apart, and how high the canopy stands.

A desert has its landforms: how high its dunes stand, how far apart, and which way Tower blows the sand; how much of it is rocky hills; how tall its plateaus are, how much of the ground they cover, and how many cliff bands they are cut into; and how much past water Tower quotes there — dry washes, how many salt pans, and whether they are ringed by the shorelines of lakes that never stood. The detail card says what was actually made: "24% standing rock, 37% dune field, 35% cut by washes, 4 salt pans."

A railway is a line with a line speed. Its track keeps a grade of its own over the ground rather than cutting it — the highest ground within a kilometre either side, eased — so it stands on piers much of the way and crosses the gorges on viaducts. There are two: the Axial Line, from Sisters Station through The Narrows and past the Capital to the Deepward Sea, and the Uplands Ring, round the world at 140 km, whose trains weigh 31% more running spinward and 27% less running antispinward.

A region is a label, and it can also be ground Tower raised: give it an uplift and everything inside it is lifted to that height above the open ground, wherever that is higher than what is there. The Uplands are one.

Every feature also has a blurb, shown to readers; notes, which are the author's and never reach the app — though the repository is public, so they are not secret; public, false for a feature not ready to exist in the app at all; and a seed, written once, so it generates the same forever.

Canon is 0 star slats — the hull is sealed, the inner surface is one unbroken 169,646 km², and the only views out are at the end caps. The runtime control for this is disabled and hidden (decisions.md D-033): the world is generated as one unbroken surface, so any other count draws bands over land that has none.