A procedural isometric city on a diamond tile: rivers, mountains, roads, bridges, buildings and traffic, generated from a text seed with adjustable density, map size, weather and time of day. No score, no goal — press New city for another one, or flip the model dropdown to compare it against the rougher GPT-5.5 build it replaced.
City Atlas generates a small isometric city on a diamond-shaped terrain tile — rivers, mountains, roads, bridges, buildings, trees and moving traffic — from nothing but a short text seed. Press New city and the whole thing is rebuilt in place: a different seed, a different river course, a different skyline, in under a second, entirely in your browser.
The author's own post is a before-and-after: an earlier version thrown together with GPT 5.5 was, in their words, garbage, so a single follow-up prompt had GPT-6 Astra take the same requirements and rebuild it from scratch. What makes that claim checkable rather than just asserted is that both versions ship in the same page — the model dropdown in the header switches live between the current renderer and the author's preserved earlier build, so the comparison the post describes is something you can actually toggle, not just a video to take on faith.
The public repository (CatsJuice/random-city) confirms what the network traffic already shows: this is a React and Three.js app that generates cities from seeds entirely on the client, with a Web Worker doing the layout math and Kenney's CC0 model pack supplying the buildings, trees and vehicles. Nothing about generating, naming or exporting a city touches a server.
How to play City Atlas
- Give it a moment on first load — the page downloads Kenney's building, tree and vehicle models and shows a brief "Preparing lights and shadows" progress bar before the diorama settles.
- Drag anywhere on the city to orbit it. The bottom toolbar adds a top-down view, a dedicated rotate and pan mode, zoom in/out, reset view, an auto-rotate toggle, and Export city image, which saves a screenshot of the current framing.
- Press New city for a fresh seed and layout. City settings (the sliders icon) exposes the seed as an editable text field — so a specific city can be shared or reproduced by typing its seed back in — plus building density, map size (Standard / Large / Expansive), weather (sun, rain, snow), the time of day and how fast it advances, and an interface language picker with ten options.
- The dropdown in the header, labelled with a model name, is the comparison the author's post is about: GPT-5.6 Astra is the current renderer this page opens with; switching to GPT-5.5 swaps in the author's earlier, rougher build — a flatter Canvas-drawn city with compass labels and a plain sun icon, and a shorter toolbar with no top-down view or auto-rotate. It is a real second implementation living in the same bundle, not a filter on the same scene.
Click Fullscreen above the game for the largest play area. If the game does not load, use Open in new tab to play it directly from our copy.
Why it's worth your time
Every generation step is local and seeded, not fetched: watching the network tab through a full session — page load, New city pressed repeatedly, both renderers switched between — turns up nothing but the initial model and texture downloads. The seed shown in City settings (e.g. SHIO-2048) is the entire recipe for a given city; typing it back in rebuilds the identical layout.
The footer quietly reports what got built — a buildings/trees/bridges count for the current city — which is a nice, honest touch for something that could easily have hidden its own mechanism.
The GPT-5.5 toggle is the most interesting design decision on the page: most projects that get rebuilt just replace the old version. This one kept the old one running, in public, specifically so a visitor can see the gap the author is talking about instead of trusting a screenshot of it.
Our mirror is a modest ~7.5 MB — light for a 3D scene — because the geometry is a few dozen reused Kenney models instanced many times over, not per-city custom assets.
How it was made and where it lives
Hosted on the author's own page. The author publishes as cats_juice. Original: https://city.oooo.so. The Play button above runs our own unmodified copy, so the game keeps working here if that page goes down — which on free hosting usually has nothing to do with the author. Nothing about the game itself is changed, and the original stays linked from every page.
The post that led us here
CatsJuice@cats_juicePulled out the garbage I threw together with GPT 5.5 before — one sentence had GPT-6 Astra reorganize the requirements and reimplement it from scratch. Slumped back in my chair. This is a comparison video.
If you like City Atlas, try these
Orbital Garden is the other seed-driven, no-objective sandbox here, particles instead of city blocks. Tavora is the closest in subject — another 3D place built from real-world geography — though it's one fixed island rather than an endless supply of new ones.
Frequently asked questions
- Is City Atlas a game?
- No. There's no score, goal or way to lose — you generate cities and look at them from whatever angle you like.
- Does "New city" still work in this offline copy?
- Yes. City generation, the seed, weather, time of day and both renderers all run client-side from a text seed — nothing here calls out to a server, so the mirror regenerates cities exactly like the live site does.
- What's the GPT-5.5 option in the dropdown?
- The author's own earlier build, the one the source post calls "garbage" and rebuilt from scratch with GPT-6 Astra. It's kept live in the same page as a direct comparison, and our copy captures it too, so switching to it works offline here as well.
- Is the source available?
- Yes — CatsJuice/random-city on GitHub, linked from the game page and from the app's own GitHub icon.
- Can I keep an image of a city I like?
- Yes. "Export city image" in the toolbar saves a screenshot of the current view; there's no way to save the seed and settings as a shareable link, only the seed text itself, which you'd need to copy down yourself.
- Is City Atlas free to play?
- Yes. Everything on Floating Arcade is free, with no account, ads or downloads.
- Can I embed this game on my own website?
- Yes — copy the snippet below. The embedded player links back here, which is how we ask to be credited.
Embed City Atlas on your site
Paste this where you want the game to appear:
