Commander Keen 3: Keen Must Die!

Galactic alphabet

In the schoolhouse in the Secret Level, you can see the English to Standard Galactic alphabet translation! This can be used to translate the signs and markers in the game.

Messie

Messie, the Loch-Ness-like monster, tours the globe constantly, but she's hard to see. If you find the area on the world map where you can get close to water, wait there until Messie shows up and she'll take you to the Secret Level.

Trilogy

Keen Must Die! is the third of three episodes of the series Commander Keen: Invasion of the Vorticons.

Information also contributed by John Romero