OpenRA

OpenRA is a free, open source real-time strategy game engine that reimplements the early Westwood Studios classics. It includes three bundled mods — Red Alert, Tiberian Dawn, and Dune 2000 — that recreate the original gameplay of the corresponding commercial Command & Conquer releases, with a modern open engine underneath.

Unlike the original 1990s games (which run through DOSBox and are no longer sold individually) or EA’s paid Command & Conquer Remastered Collection, OpenRA is fully open source under the GPL-3.0 license. The engine is written in C# on .NET with SDL2 and OpenGL for rendering, runs natively on Windows, Linux, macOS, and *BSD, and includes a built-in map editor, a Lua scripting API for missions and unit logic, and support for community-made mods and maps.

Online multiplayer is supported out of the box via the GameServer lobby, and a dedicated server mode is available for hosting persistent games. For players who want to play the classic C&C-style RTS experience without paying for the Remastered Collection or hunting down old CDs, OpenRA is the most active and complete open source option.