Enter Password" at the Options menu and input one of the following passcodes to gain access to all of the game's stages, including the endings:
Easy Difficulty Level:
X, O, X, O, X, Triangle.

Medium Difficulty Level:
X, X, Triangle, O, X, Triangle.

Hard Difficulty Level:
Triangle, Square, Square, Square, X, Triangle.

Passwords

level 1: (NONE)
level 2: X, Triangle, X, O, O, O
level 3: Triangle, Square, X, O, O, X
level 4: Triangle, O, X, O, O, Triangle
level 5: Triangle, X, X, O, Square, Square
level 6: Triangle, Triangle, X, O, Square, O
level 7: Square, Square, X, O, Square, X
level 8: Square, O, X, O, Square, Triangle
level 9: Square, X, X, O, Triangle, Square
level 10: Square, Triangle, X, O, Triangle, O
level 11: O, Square, X, O, Triangle, X
level 12: O, O, X, O, Triangle, Triangle
level 13: O, O, Triangle, X, X, Square
level 14: O, Triangle, X, O, X, O
level 15: X, Square, X, O, X, X
level 16: X, O, X, O, X, Triangle

Thanks to Revolution reader Jason Sommerfeld!