Wandering City Mechs

Long after the fall of civilization, the great powers of the old world survive only as mobile war-cities massive armored metropolises sealed beneath shimmering, indestructible dome shields. Some glide across the sky on anti-gravity engines. Others march on the backs of titanic mechs. And the rest, like predators of steel, crawl across the wasteland on colossal treads that crush mountains beneath their weight. You either kill other city's or assimilate them into yours.