In Infested Planet, Alex Vostrov is taking the basic gameplay dynamic he explored in his freeware Attack of the Paper Zombies and running with it. As in the previous game, you control a handful of space marines who must clear out a succession of spawn points that churn out huge numbers of opponents -- Zerg-like bugs here rather than the zombies of the previous game -- but Infested Planet has many improvements.
In Infested Planet, most levels are algorithmically generated rather than predrawn. There are several different weapons with which you may equip your marines, each with its own advantages and drawbacks; in addition, they can construct a number of defensive turrets, and use grenades to wipe out enemy structures. In early levels, the bugs have only one basic attacker, plus a defensive turret and eggs that can spawn more bugs, beyond those that bug bases create; later on, they can have a wider variety of building and unit types.