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The mages in the second game were pretty good, I think. Although, the presence of pigs there... Well, other than that, it was fine.
The Tower in the third game is an immortal classic, of course. I think many here know everything about this Castle inside and out. Having gremlins in the city is more interesting than having halflings in the second game, and the genies are even better integrated, without which this Castle is simply unthinkable. As for the nagas... The presence of snakes in the snow Castle is a debatable phenomenon, but the unit is interesting.
Heroes 4, Order. I don't particularly like it. They gathered halflings, nagas, and even gnomes for some reason; in general, it has all the shortcomings of the mage Castles from previous series, except for the pigs. However, I think the dragon golems are a successful unit, better than the titan, as they are more unique.
Heroes 5, Academy. The mages were moved from the snow to the desert and given a distinct cultural style, which is reflected in the architecture, the appearance of the units, and the heroes. This is definitely a positive aspect. However, in other respects, the Academy is a clone of the Tower from the third game, except that the nagas have turned into rakshasas, which I also consider a plus (on the other hand, rakshasas are very similar to nagas from the third game from a tactical point of view, so it's debatable).