Commander Keen 2: The Earth Explodes

Commander Keen follows the Vorticons home to their mothership, orbiting Earth and aiming a network of giant Tantalus Ray cannons at the planet's greatest landmarks. Keen must sneak aboard and sabotage each cannon before the Vorticons blast the cities below.

The second episode of Invasion of the Vorticons (Apogee, 1990) was sold only in the registered trilogy. It trades the open Martian cities of episode one for the tense corridors of an alien warship.