You need 2 keys and 2 locked treasure chests. As you open the first chest, hold down the 'Help' key, collect the treasure. Open the second chest while holding down 'Insert' ('0' on the key pad). An Ankh (golden cross) will appear and you must collect it. You'll hear a gong. You should now have 50,000 health points and 32,000 reflective super-shots. The shots will vanish once you leave the level, and if you pick up an amulet, the counter will be reset to 10 shots.

To avoid tricky levels, pause the game and wait for three minutes. Press 'Fire' and all the walls turn to exits. This happens anyway, but if you pause the game, you don't lose energy, and won't get attacked while waiting three minutes.