Hold [Shift] and press [?] during game play. 
Enter one of the following codes in the black box.

Effect                           Code 
--------------------------------------
5 Gems                         - gems
All keys                       - keys
Clear surrounding four squares - zap
Ammunition                     - ammo
Debug mode on                  - +debug
Debug mode off                 - -debug
Full health                    - health
Gain 3 torches                 - torches
Darken room                    - +dark
Lighten room                   - -dark
Sets the flag flagname         - +flagname
Clears the flag flagname       - -flagname