banteg/crimson

Crimsonland, rebuilt

Crimsonland (2003), as its 1.9.93 GOG "Crimsonland Classic" release (2011), rebuilt twice:

• A matching decompilation: C/C++ source that Visual C++ 6 compiles back into the original crimsonland.exe and grim.dll, instruction for instruction. Compiled to WebAssembly, it plays the original game in your browser.
• A reimplementation in Python and raylib that matches the original's timings, random rolls, float32 rounding and quirks tick by tick, and adds what a modern release needs.

Play in your browser · Leaderboard · The full story · Docs · Changelog · Telegram group

Play

crimson.land/play is the original game, built from the recovered source. There is nothing to install: it downloads the game files on first visit, keeps your saves in the browser, and plays with a keyboard and mouse or a gamepad. The original's documented bugs are fixed, and the Ranked box in the Play Game menu enters runs on the leaderboard. Play co-op gives you an invite link: send it to a friend and play together from your own browsers. In a browser with WebGPU, the RTX lighting box in Options turns on RTX lighting.

The Python port plays the same game with more room around it: any resolution with borderless fullscreen, local co-op for up to four players on their own controllers, the original bugs back on request with --preserve-bugs, and a crimson CLI for replays and quests. Install uv, then:

uvx crimsonland@latest

Its player guide covers controls, options and commands.

A native desktop client runs the browser build over SDL3 on macOS and Linux (build notes).

The feature matrix compares the three. The original Crimsonland Classic assets are distributed with permission from 10tons, and every client downloads them on first launch.

Beyond the original

• Sharper art. The original shipped its textures as JAZ files, JPEG color with a separate alpha channel. The game files every client downloads carry the uncompressed source art from 10tons instead, free of JPEG artifacts, wherever it matches what shipped.
• RTX lighting. An optional look that lights the battlefield with the game's own light sources. Muzzle flashes, Fire Bullets and Ion beams, plasma and explosions light the wet ground, the creatures and the player, and cast soft shadows. Smoke is a fluid simulation: fresh explosion smoke glows hot and cools to gray, and creatures stir it as they run through. Monster Vision shows heat. Light adds up as light does under one tonemap, so a screen full of fire keeps its color instead of clipping to white, and on an HDR display highlights go past it where the client can. Every client has it, behind the RTX lighting box in Options; see the RTX rendering guide.
• Deterministic runs. The simulation steps at a fixed 60 Hz from a seed and the recorded inputs, so a run plays out the same way every time, on every client.
• Online co-op. The original's two-player mode, over the internet: one player opens a room, the other follows the invite link, and both play Survival or any quest from their own browsers. Rollback netcode keeps each player's aim responsive under lag, and a relay steps in when a direct connection can't be made. See the network co-op specification.
• Replays. Every run is recorded. Watch any of them from the high score screen or a link, seek anywhere, and with the Python port, verify one tick by tick or render it to 60 fps video.
• A verified leaderboard. crimson.land ranks Survival and every quest, normal and hardcore, for one player and for online co-op pairs. A score is a whole run, and the server replays it on the recovered original code before it counts. There is no sign-up: each client makes a key on first launch. Bots and tool-assisted runs are welcome on their own board.
• Custom quests. Crimsonland now comes with a Crimsonlang. Write a quest in it, install the quests extra (included in repository development installs), play it with crimson quest play, and publish it to get its own page and leaderboard on crimson.land/custom. An LLM can write one from a single-file spec.

Inside

The two rebuilds keep each other honest. The Python port settles open questions against the original code running under an emulator, and CI requires the recovered source to play every recorded run exactly as the port does, under both the original rules and the fixed ones.

Decompilation

The matching decompilation of 1.9.93 is complete: every game and engine function comes from recovered source, with every reference checked. Bundled third-party libraries (D3DX8, the MSVC runtime, the image and audio codecs) keep their upstream provenance and are not counted.

The same source tree builds other releases, each with its own compiler profile and CL_BUILD define, and decomp.dev tracks each from its own evidence:

builds.json pins and maps more builds than are reported.

Developing

CONTRIBUTING.md covers the project layout, the reverse-engineering and parity workflow, and the checks to run.

Legal

This project is an independent reverse engineering and reimplementation effort for preservation, research, and compatibility. Original Crimsonland Classic assets are distributed with permission from 10tons; the game code and reimplementation remain independent.