OpenRA

OpenRA is a project that recreates and modernizes the classic Command & Conquer real time strategy games. It's a flexible open source game engine (the OpenRA engine) that provides a common platform for rebuilding and reimagining classic 2D and 2.5D RTS games (the OpenRA mods). This means that OpenRA is not restricted by the technical limitations of the original closed-source games: it includes native support for modern operating systems and screen resolutions (including Windows 10, Mac OS X, and most Linux distros) without relying on emulation or binary hacks, and features integrated online multiplayer. The game engine is open source, but it downloads the freeware versions released by EA.