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In my opinion, the Hydra should be removed; it doesn't fit the Castle at all. First, Hydras live in swamps, and second, it doesn't fit the Castle's theme due to its slowness and bulkiness. For the Gnomes, give them a speed rune. Replace the Hydra with a Manticore (not the neutral ones, of course) with appropriate stats, or Medusas (with a petrification or debuff ability... preferably with good ranged attacks). Accordingly, make the Assassins ranged units, even though Mages already have 3 archers, one of which is level 7. As it stands, the Storms with their Thunder Elementals are overpowered; a single dragon flies around, and the archers aren't even comparable to other Castles.
Minotaurs fit the Castle well, but their stats are lacking. Instead of "Bravery" (which is an unnecessary ability, in my opinion), it would be better to give them a +1 bonus to morale, like in Heroes 3. Then, either give them the speed of Orcs (6, I think) and a similar attack system (not 2 weak attacks, but 1 normal attack and a chance for 2), or give them something similar to the Orcs' second upgrade, or like the Armored Knights with protection from ranged attacks, so they would actually be useful. In general, I think the stats and abilities of Orcs from level 3 (excluding Rage, of course) would suit them very well.
P.S. Making phantoms out of the current Minotaurs seems pointless to me. What's the point? They'll just get close and be killed. They won't even do anything, like a Czech. It would be better to make phantoms out of Furies, Matriarchs, or someone else.
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