INTACTO - I'm glad that this method also worked. Theoretically, I assumed that someone would use it =)
jjjoker777 - on hard difficulty, you can't amass an army anyway. Although, your option sounds good. I think we'll do it that way.
Regarding the spells - there are a lot of them, I don't know why the companion hero doesn't learn them. When I went to him, he had all the towers built, I don't know why he doesn't use them.
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By the way, I also completed it normally. I decided to level up a ballista from the very beginning, since I found the Ring of War Machines. In the end, the ballista could eliminate 1k level 1 creatures per attack series.
I even took a screenshot for memory (I even managed to break through the Haven's gate in its original form).