On the first level, climb the left side of the first tree you come to, climb to the very top. Now press 'P' to pause the game and type ''ACEVIEWFROMUPHEREMATE'' and the screens border will flash.

Unpause the game and use the following keys:

Code - Effect
''1-6'' - go to the corresponding level with infinite energy.
C - Infinite Continues
W - Maximum weapon configuration
E - Takes you directly to the end of level guardian.