2K Games and Firaxis have jointly announced the release of the official Software Development Kit (SDK) for the PC strategy title Sid Meier’s Civilization IV. The SDK contains the core game DLL source code, including the game and A.I. code, and allows players to completely rewrite or modify their own Civilization world. The release opens the door for the mod community, as it allows for the creation of an completely new game that is built around the Civilization IV mechanics, rather than simply a a slightly modified way to experience the same game. In addition, the companies also released the completed Persistent Turn Based Server (Pitboss) application, which is a dedicated multiplayer server for online and local games that offers players the ability to log in and continue progress in a game at any time. A beta version of the application was released in January. Lastly, patch version 1.61 was released today as well, which includes numerous changes, additions, and fixes for Civilization IV. For a full list of features impacted by the patch, visit the game's website.