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The game also wouldn't let me attack that priest. But I was persistent, and after the 2nd or 3rd attempt, I managed to do it. It seems I directed Zehir not at the priest, but at the Castle.
If I had taken the army from Wolfshtein and/or arrived with a full mana supply, I would have definitely defeated him. Of course, this would have ended badly, just like in the 5th mission of the 1st Campaign of the original game. There, I broke through the garrison and captured the demon Castle.
The Dwarves haven't mastered teleportation yet. I just asked if they would come to the surface. I didn't know that the Dwarves couldn't bypass the priest either. Now I know, and I can calmly, without fearing a Dwarf raid, go and get acquainted with other enemies.
In the fifth mission of the first Campaign, I somehow broke through the garrison as an experiment, took the demon Castle, and destroyed Agral. But this didn't change anything. Nicholas ran to the place where Agral used to attack him. After that, there was a cutscene where he was killed, and the mission was completed.
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Moving the flying city in the Dwarf Campaign simply costs 20,000 experience points, which will be useful in the next Campaign