Pause the game, then hold L + R and press A, B, A, A, B, B, A, A. You will advance about 5 levels, depending on where you are.
All weapons, items and keys
Pause the game, then hold L + R and press A, B, B, A, A, A, A, A.
Berserk mode
Pause the game, then hold L + R and press B, A, B, A, A, A, A, A.
Computer map
Pause the game, then hold L + R and press B, A, A, A, A, A, A, A.
God mode
Pause the game, then hold L + R and press A, A, B, A, A, A, A, A.
Invincibility
Pause the game, then hold L + R and press B, B, B, A, A, A, A, A.
Radiation suit
Pause the game, then hold L + R and press B, B, A, A, A, A, A, A.
Secret Military Base Level
On Level 3, go out of the room you start in, go left and through the door at the end. Take the right passage and then the first left.
Go down the stairs, flick a switch and then head back up to the door. You will see a third passage. Go through, into a room. Kill anything and destroy every barrel before leaving. Then quickly head back in, go right and then left. You will see an opening dead ahead.
Go through that opening and until you get to a platform with a rocket launcher. Then go into the acid to your right, through a passage and out to a lift.
Return to the place you spawned and you should see a new bridge. Follow the per-defined route to find a switch taking you to the military base.
Tower Of Babel level
Pause the game, then hold L + R and press A, B, A, B, A, B, B, A.
Advance to anonter level ...