Pause the game. Hold R and press Down, Up, , , , , Down, Up, , . 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, , , , , . 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!