Wolfenstein: Enemy Territory (also referred to as simply Enemy Territory, ET or WET) is a free multiplayer first-person shooter video game set during World War II.
It was originally planned to be released as a commercial expansion pack to the popular FPS Return to Castle Wolfenstein (also referred to as RTCW) and later as a standalone game. However, due to problems with the single-player aspect, the multiplayer portion was released on May 29, 2003 as a freeware standalone game.
In early 2004 the source code for the game logic (not the game engine) was released to the benefit of its modding community. The game uses a modified RTCW engine, itself being a modified Quake III: Team Arena engine (Id Tech 3, which has been open-source since 2005).