Counter-Strike: Condition Zero is a first-person shooter game developed by Ritual Entertainment, Turtle Rock Studios, and Valve. First released in 2004, the game used a three-dimensional video game engine known as GoldSource to enable multiple players to join in the game. Condition Zero also features updated character models, textures, maps, and effects of the original Counter-Strike. In addition to being a multiplayer game, it also features 2 single-player campaigns: Tour of Duty and Condition Zero: Deleted Scenes. Originally, you can only play Condition Zero on Windows XP or newer but now you can play on both Windows and Mac OS X computers.