Zero-K is a free multi-platform open source real-time strategy video game. Initially based on content from Total Annihilation on the open source Spring Engine, it was forked and all proprietary content replaced, and evolved into a completely new game with unique features. Among the games powered by the Spring Engine, Zero-K makes extensive use of Lua scripting for interface and gameplay changes/enhancements, as well as unique real-time strategy concepts such as a flat technology tree.