Valve has published the source code for Terror Strike, an early prototype for Left 4 Dead. The publication may have been accidental and occurred via an update to the first Counter-Strike game. A mod has now been released to run the prototype.

It is a prototype game with its own folder: zombie_city. The game mode uses a lot of Counter-Strike assets, such as weapons and characters. Terrorists have to plant a bomb in an urban environment, while anti-terrorist zombies appear here and there and the player tries to confront them with knives. The player must shoot these zombies, after which they come back to life. Once the player plants the bomb, more zombies will appear. The One Epicplayer: Blue Shift on YouTube shows how the game mode works and He also explains How players can experience the mode.

The source code appeared last week in an update for Counter-Strike, in honor of Valve’s 25th anniversary. This is not the first time parts of Terror Strike have been leaked, this also happened earlier this year, and the zombie_city map was created based on this leak. This mod is for Counter-Strike: Source and the new mod is for Counter-Strike: Condition Zero. Left 4 Dead is produced by Turtle Rock Studios.