Pause the game, and go into the options menu. This is called the Fortress of Solitude, enter these codes anywhere in the menu. Try to enter them at a fairly quick pace.

Effect - Code

All Unlockables - Left, Up, Right, Down, Circle, Square, Circle, Up, Right, Square Unlock
Infinite Metro Health - Circle, Right, Circle, Right Up, Left, Right, Circle
Unlock All Power-ups - Left, Circle, Right, Square, Down, Circle, Up, Down, Square, Circle, Square