Press L & R in Pause Menu, then press he following buttons to activate the cheats

All Weapons, items and Keys:
A,B,B,A,A,A,A,A

Berzerk Mode:
B,A,B,A,A,A,A,A

Skip to Level, Tower of Babel:
A,B,A,B,A,B,B,A

Computer Map:
B,A,A,A,A,A,A,A