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Firstly, it's best to play as the Barbarian (the most optimal choice, as I personally completed the game with him, and I'm definitely not a super player). Secondly, you need to go through the dungeon and collect all the creatures that are guarded in improvised cages (it's a dungeon, so the prisoners will help). With the default army, you obviously won't be able to pass the gate, and even with that army, it won't be very easy...
