Added after 13 minutes
Arantir (Vitalios) is wrong. You can complete this mission without capturing the castle near the witch. He didn't bother me, and I could move around his territory without any problems. And I didn't need much steel either.
Added after 8 minutes
Who captured the castle that Arantir (Vitalios) was talking about, where the danger to the hero is high? (I barely made it through, and I had only 3 zombies and 7 skeletons left).