Steel Reign

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

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

Super Tank and More:
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.