LEVEL SELECT:
At the title screen press S,S,S,S,S,T,T,T,T,T,T,T,O,O,O.

INVINCIBILITY:
Pause the game and press T,T,X,X,O,S,O,S,D,L. You should hear a sound, 
input the code again will turn it off.

MAXIMUM POWER:
Pause the game and press U,U,D,D,L,R,L,R,X,O. You should hear a sound, 
you can also repeat this code at any time during the game.

Hidden Level:
First, select Original Parodius. Destroy the entire first wave of enemies, but on 
the second, shoot only the very first enemy. Wait around 20 seconds without 
shooting anything else. All of the on-screen enemies should dissapear, and 
you'll be taken to a hidden stage resembling a subway station.