There are several cheats for this, that consist of
a command-line parameter and an ingame key :

Parameter Key Effect
----------------------------------------
KmCiNpOcKeT [F1] Skip level
Nicole [Shift]+[P] Walk thru room
MEANING [Shift]+[T] Extra energy
Donkey [T] Extra treasures
Grumpy [K] Extra keys

All these parameters are case-sensitive and can be
combined as you like.