Type JOSHUA and you'll get a reply...
 
 Music test:
 Press M on the opening screen and you get a jukebox...


 Full health, ammunition; all weapons, keys: 
 Press L + I + M during game play. 
 Note: Your score will reset to zero after this cheat is enabled. 


 Commander Keen cheat message:
 Press A + B + T during game play. 


 Invincibility:
 Press [Tab] + G + [F10] during game play. 


 Cheat mode:
 Start the game with the -debugmode command line parameter.
 Press [Left Shift] + [Left Alt] + [Backspace] to enable debug mode. 
 Enter one of the following codes to enable the corresponding cheat function.

 
 Cheak keys (press TAB with key):
  B Change Border color
  C # of doors, actors, food, ect
  E End level, advance to next
  F Find x, y coordinates and angle of you
  G God mode!!! (invincibility)
 ÿ  (Some versions you have to press F10 as well.)
  H Hit yourself
  I Add items to you
  M Memory useage
  N Clipping toggle (walk through walls) [v1.0]
  O Overhead map [v1.0]
  P Pause game
  Q Quick exit [may crash sod]
  S Slow-motion mode
  V Add VBLs [v1.1+]
  W Warp to level of choice
  X Add extra stuff [v1.1+]