Widelands is an open source (GPLed) real-time strategy game. It is built upon the SDL and other open source libraries and still under development. If you knew Settlers I & II (© Bluebyte), then you already have a rough idea what Widelands is all about. Yes, it is where the inspiration for Widelands came from... Play it on Mac OS X/Linux/Win against human & AI opponents.
Mall Creeps
