To enable cheat mode, start a new game and stop your carpet. Press I and type "QUICK", then press Enter. To use a code, press any of the following while playing.

Code       Effect
ALT-F1     All the spells
ALT-F2     More Manna
ALT-F3     Destroy all players
ALT-F4     Destroy all balloons
ALT-F5     Destroy all castles
ALT-F6     Heal
ALT-F7     Destroy all creatures 
SHIFT-C    Completes the level