Pause the game. Hold R and press Down, Up, Square, Triangle, Triangle, Circle, Down, Up, Square, Triangle. Now when you pause the game a cheat menu will appear with invincibility and all weapons options.
All Keys and Artifacts:
Pause the game. Hold L + R and press X, X, Circle, Square, Triangle, X. An 'Artifacts and Keys' option will appear in the menu.
Rescue the farmers in the Haunted Ruins level, and they will lead you to a stash of gold. Save and reload the game, and the gold is back for the taking. Repeat as desired.
Thanks to Revolution reader Jonathan and bonecontrol!