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I immediately left the required army – thanks for the tip about the "course" of emeralds to greens... I hired two elves in the Castle, and one went with me to meet his fate. The little elf barely managed to defend himself the first time. But then the purple one arrived only two weeks later, and the Castle was already fully built. So the weak elf leveled up every week, and by the end, he was already level 20. And with Fedei, I captured the Forest Alliance Castle on the fly, hired a weak hero, fought twice, developed logistics there, and Fedei went deeper towards the Necropolis. I met a Necropolis hero, defeated him (the danger was very low), and then barely made it through two outposts. I captured the Castle as if it were nothing, went down, freed the redhead aunt, and upstairs, freed the redhead uncle for the angels. The weak hero delivered a triple increase from the second city to Fedei once and to the main Castle once. As soon as I freed the redhead uncle, the mission ended. It was unexpectedly quick. I thought I would have to fight someone else.
By the way, it's nonsense that you need to eliminate other heroes besides the redhead uncle and aunt from the prisons. I had three alternative heroes.