Use the following cheat codes (hold Shift and press the indicated arrow keys:
God Mode: Right, left, right, left, up, down, up, down
10,000 Silver and Adderstones: Up, up, down, down, left, right, left, right
100X Damage: Up, down, up, down, left, right, left, right
Invincibility: Right, left, right, left, up, down, up, down
Unlock Levels: Right, right, left, left, up, down, up, down
Full Health: Left, left, right, right, up, down, up, down