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Enter the following cheats on the password screen:
Full Metal Jacket - Square, Circle, X, Triangle, Triangle, X, Circle,Square.
Invincibility - Square, Triangle, Circle, X, X, Circle, Triangle,Square.
Enable Port 2 - X, Triangle, Square, Circle, Triangle, Circle, X,Square.
(During play, press Triangle on pad 2 to destroy the enemy base, or press Square to destroy your own base.)
Passwords:
Level 2 - Square, Triangle, X, Circle, Triangle, Square, Circle, X.
Level 3 - Square, X, Circle, Triangle, Triangle, Square, X, Circle.
Level 4 - Square, Triangle, X, Circle, Circle, Triangle, X, Circle.
Level 5 - X, Circle, Square, X, Triangle, Square, X, Circle.
Level 6 - Square, Circle, Triangle, X, Circle, X, Square, Circle.
Level 7 - X, Circle, Square, X, Triangle, Triangle, X, Circle.
Level 8 - X, X, Circle, Triangle, X, Circle, Square, X.
Level 9 - Circle, X, Circle, Square, X, Circle, Square, Circle.
Level 10 - Circle, Triangle, Circle, Circle, Square, Triangle, X, Circle.
Level 11 - X, Square, Circle, X, X, Square, Square, Circle.
Level 12 - X, X, Circle, X, Circle, X, Square, Circle.
Level 13 - Circle, Triangle, X, Circle, Triangle, Square, X, Circle.
Level 14 - Square, Triangle, X, Circle, Triangle, X, Circle, Square.
Level 15 - Square, Triangle, X, Circle, Square, Triangle, X, Circle.
Level 16 - Circle, X, Circle, Square, Square, X, Square, X.
Level 17 - Circle, Square, Triangle, X, Triangle, X, Circle, Square.
Level 18 - Square, Circle, Square, X, X, Circle, Circle, Triangle.
Level 19 - X, Circle, X, Circle, Triangle, Triangle, X, Circle.
Level 20 - Triangle, Triangle, X, Circle, Triangle, Triangle, X, Circle.