0 A.D. is an unusually fancy RTS game featuring 3D graphics. Well, “unusually” for libre games.
It’s in development since 2000, still not considered “finished” as of 2018, and warns about bugs and such, but it is pretty usable nevertheless: probably more so than some of the “finished” ones.
The game features 12 ancient civilizations, a few resources, a variety of units and buildings, plenty of maps, single-player and multiplayer modes, different difficulty levels, but no campaigns yet. As with other real-time strategies, the gameplay boils down to learning how to expand fast, but without getting too vulnerable to attacks meantime. It may not sound exciting, probably because it isn’t, but if you’re into RTS, it’s a fine one.
Despite nice graphics and warnings about high resource consumption with larger armies, the resource consumption with the default settings seems to be moderate, apart from quite high disk space requirements (about 1.6 GB of data files).