Enemy Nations is a free (source available on github) RTS/CityBuilder with multiplayer. It was originally released in 1996 but due to several issues, publisher and otheerwise, was quickly abandoned.

The source was released, and the community got it working on modern systems.

3.00.015 brings us -Improved traffic, AI performance and island invasions -Shift-drag queues buildings and roads -Fixed saved multiplayer joining and several crashes -Loose game data for easier modding; artwork/audio mods remain multiplayer compatible -Warehouse controls, foraging and edict updates -Resonance Sweep reveals one enemy rocket per 30 real seconds at full power -UI and animation fixes