Pause the game play by pressing F1 and then type in RHIANNON and then unpause the game play. You can now use the following keys to activate the cheat you wish for.

Note. If any cheat fails to activate just pause the game and enter the code again

Code - Effect

Press [F6] - Warp to the shop
Press [F7] - Skip the current level
Press [F8] - Max keys & magic / $10,000 Cash
Press [F9] - Invincibility On / Off