To activate cheat mode, press I, then type "ratty". To use a code, press any of the following while playing.

Code       Effect
ALT-F1     all spells
ALT-F2     more mana
alt-F3     eliminates all players
alt-F4     eliminates all castles
alt-F5     destroy all balloons
alt-F6     full health
alt-F7     kill all creatures
SHIFT-C    level completed
CARPET-#   skip to level #

Note: On some versions of the game you must type "quick" instead of "ratty".