On the main menu, click the left joystick (L3) then press BLACK, WHITE, YELLOW. (Unverified)

Submitted by zach lyter

Alternately -- WHITE, BLACK, BLACK, WHITE, BLACK, BLACK, WHITE. If entered correctly, down in the bottom left hand corner of the "Main Menu" screen will say "Cheats". Start the game, pause it, then enter cheat screen. (Unverified)

Submitted by Beth A. Eickhoff

In Valley of the Kings destroy all of the statues to reveal a hidden level.

To unlock Mental Mode, complete the game on Serious Mode. Mental Mode makes enemies phase in and out.