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I defeated Gunnar without any losses, but using a very strange tactic. I stayed on the defensive in the castle, the medusas fired, Gunnar broke down the lower wall with an earthquake, I sent the summoned air elementals down immediately after the moat, Gunnar climbed into the moat with nagas and defended himself there until the end of the battle, and the harpies destroyed his entire army in the meantime :confused:
Towards the end of the game, wizards become significantly more important; it seems you can complete the game without harpies.