For Super Shields (invincibility), press L2, L1, R2, Circle, Square, Circle, Circle, L1, L2, L1 at the main menu.

To access a secret level, press L1, L2, L1, L2, R2, R1, Square, Circle, Square, Square at the main menu.

At the main menu, press L1, L2, L1, Circle, Square, Circle, Circle, L2, L1, R2. This code unlocks a Super Tank with unlimited weapons, and also provides access to all of the other tanks.