Perimeter is a real-time strategy game from the Russian studio K-D Lab, first released in 2004. Bases are protected by energy shields, the perimeters of the title, and much of the fighting is about claiming and defending ground. You can reshape the terrain itself, raising and flattening land to make room for buildings, and your units can transform into different forms to suit the battle at hand.
There is a single-player campaign with a science fiction story, plus skirmish and multiplayer matches against AI or other people. It suits RTS players who want something less conventional than the usual base-and-army formula and who enjoy managing energy and territory.