Firstly, we are talking about the confrontation between Stronghold and Dungeon. If the opponent has already started with these heroes, how will you recruit them? Besides, it's unlikely anyone would argue that Stronghold doesn't contribute to the development of magic: the Grand Mage is limited to level 3, Valhalla only gives +1 Strength, etc. Dungeon, on the other hand, has a focus on using magic - the Whirlpool, for example, even without upgrades, allows Dimer or Zhedditt to cast their spells twice, which gives them a huge advantage at the very beginning. There's also the Military University, +1000 Experience, leveling up the hero, and for a mage, this almost always means +1 either to Knowledge or Magic Power. Secondly, in my opinion, the mages of Dungeon have better stats compared to the barbarian mages.