Extra hard difficulty:
Hold Left for three seconds, then press Right at the main menu. Next, press Circle, Triangle. The "Option" selection will turn red to confirm correct code entry. Enter the option menu to change the difficulty setting to extra hard.

Play as the enemy:
Press Right, Down, Down/Right, Down, Down/Left, Left, Triangle at the main menu. The "Game Start" option will turn red to confirm correct code entry. Select "Game Start" to access a menu that will allow all bosses and some enemies to be selected for game play.

Versus mode:
Press Down, Circle, Right, Square, Up, X, Left, Triangle at the main menu. A sound will confirm correct code entry. A versus mode option will be now be available.