Counter-Strike: Condition Zero is a first-person shooter video game developed by Ritual Entertainment, Turtle Rock Studios, and Valve, and published by Sierra Entertainment and Valve. The follow-up to Counter-Strike (1999), it was released in March 2004 for Windows. Condition Zero utilizes the GoldSrc engine and has a multiplayer mode, which features updated character models, textures, maps and other graphical tweaks. It also includes two single-player campaigns. (wikipedia.org)