Really? I didn't know that. I'm not an expert on the third game either; I've played it a couple of times, but I always thought that Fortress wasn't the strongest faction. I'm talking about the original third game with its expansions. Maybe someone can enlighten me: what makes Fortress stronger than Castle?
My opinion is the opposite. In the third game, critical imbalance is excluded by the equal availability of magic.
In general, in the third game, mage heroes are considered weaker than warrior heroes, and the most widely used spells are, surprisingly, level 1 spells.

Барон "Vampires"
Отдай свои прошлые сны,
В сомненьях себя обрети,
В бешеном танце душа кружится!
Не думай, что будет потом,
Здесь и сейчас мы живем,
Стань хоть на миг сам собой, слышишь!