To get access to the cheatcodes, pause the game by pressing Esc and hold F12 and press at the same time one of the following codes:

Up,Up,Up,Left,Left,Left,Left,Left: Full health
Up,Up,Left,Left,Right,Right,Left,Left: Full Sword Power
Up,Up,Left,Left,Right,Right,Left,Left: Skip level

Let go of the F12 key, choose continue and press the Space bar.