Aegis of Earth: Protonovus Assault is set 50 years after the "Silent Apocalypse", a catastrophe that brought monsters to the world, seeking only to kill remaining humans. These monsters gather in a few cities powered by the new element altenite.
Between attacks, there is an unlimited planning phase where the player develops their city: buying more weapons, upgrading them, and taking care of other things like structures for refugees (a bigger population results in more tax income) or researching advanced weaponry. Later in the game, the player takes control over multiple cities, including jumping between them via a world map.