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In my opinion, this is the most difficult mission in the Necropolis campaign, especially when played with a hero. I didn't complete it on my first try. In the first week, I captured two skeleton dwellings and one zombie dwelling. I recruited two groups of units south of the first town and on the first day of the second week, I captured the town.
Next, I used a bug – if the enemy sees a weak hero, they don't notice the main hero. I immediately bought reinforcements in the castle and, along with a supporting hero, went through the portal to the mine. Ornella captured the mine, and the supporting hero went to the mill. The enemy killed the supporting hero, but then Ornella caught up with him. The rest was just a matter of technique. Otherwise, it wouldn't have worked. Without the supporting hero, the enemy would run to the castle and return with a large army. Ornella couldn't handle him.
I also didn't immediately find the path to the last castle.