Commander Keen: Fall Up is KeenRush's fourth mod, made for Keen 1. The mod is all about gameplay and motion. Playing it differs largely from the ordinary Keen 1-3 because of the changes in the game's physics.
The mod is based on a system where gravity changers change the gravity, once collected. Keen has infinite lives, but not his trademark raygun or pogo stick. If player 'shoots', Keen's head will explode and he will get out of the level as in case of an usual hazard related death.
There are five gravity changers. Once Keen enters a level the gravity is always set normal (the gravity used in the original games).
makes Keen able to float in air
makes Keen able to jump higher
returns the gravity to normal
makes Keen fall slowly, and unable to jump upwards
makes Keen fall up
Groggy and confused, Commander Keen finds himself, yet again, in an unexpected zone. Where the garg am I? Why the garg am I here? Maybe he will learn...
Spoiler Warning! This shows the whole levels.