Note: these cheats worked in the demo, 
but they haven't been tested with the 
full version.
                       
Type while playing:
                         
alllevels - Show all level menus
nolevels  - Hide level menu items
destroy   - Destroy on/off
fps       - Show Frame rate
credits   - Show Credits
next      - Next Level
noammo    - Remove All Ammo
godmode   - God Mode
pause     - Pause Actors