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You killed everyone in a rather strange way, considering Kirdan is gathering an army from three castles.
The idea is that you capture the territories of the blue player and then fight Kirdan, who has a substantial army that grows over time. But by that point, you should control all the cities on the map. Of course, you can "wake him up" at an unfavorable moment, and he'll run around the map... but that means you should have been more careful when approaching the final boss.