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If you kept your key from stage 3 all the way to the end, you'll be able to use the door before the boss. If you do, you'll go to Stage 5' instead of Stage 5.

[edit] Boss: Minotaur

The minotaur is the easiest boss in the game, as you can manipulate the AI so it never attacks. Simply run to the opposite side of the room and make sure he's as far away as possible. Approach him, and attack, then back away quickly. If he hops or moves backwards, go in for another attack. Keep your distance so he won't attack.

If you're close and above him, he'll throw rocks at you. If you are close to him and on the ground, he'll strike you with his axe (he'll also attack if you attack him a few times in a row).