NOTE: This game can be played directly in Windows using a third-party engine recreation (please see the links section below).
Full gun power and keys: Press both of these keys at the same time: [Backspace]-[PgDn]. You'll get all keys and full gun power.
Debug mode: To activate, start up the game with "asp" as a command line parameter (for example "duke asp"). Then you can press the following keys at the same time during the game to cheat:
G-O-D: Full health and all items
G-O-W: Warp to the next level
Description by VGTips