Heroes 5 and 6 are not add-ons to Heroes 3. And Heroes 3 and 2 are not add-ons to Heroes 1, but they all have towers and moats. Without towers and moats, it's harder for the defenders, but easier for the attackers.
The appearance affects the first impression of the game. I think you wouldn't play if the Devils had skeleton textures and the Beserks had hydra textures.
I wouldn't judge the confrontation based on spells and other magic: after all, Life has spells that protect against all other schools of magic, and this doesn't make Life stand out among the militia. Barbarians are neutral, yes, and it's obvious that all opposing Castles are combined into one army, and the game's logic will tell you how the creator intended it. But this is a "mechanical" division. And I wrote that if Life vs. Death, Chaos vs. Order are logically perceived confrontations, then there are two Castles left: Nature vs. Barb.
Why don't I quite perceive the "mechanical" division? Because Death has two Castles that are opposite to Life and Nature - it's clear, Life has Death and Chaos - more or less clear, Chaos has Order and Life - similarly, Nature has Death and Order - it's not very clear, just like Order has Chaos and Nature... according to the mechanics, Nature and Order are opposing each other, they reduce each other's morale to the maximum, but according to "logic," I find it difficult to imagine, it's not obvious, if not downright silly...
In short, if you swap Nature and Barb, everything would fall into place. Barb fits well with Life and Chaos, and opposes Order and Death well. And Nature, by its very nature, is an excellent neutral Castle that can do without neighbors and confrontations. Let Nature keep only the Druid, and it can do without the Archer. What hero should be given to the Barbarian to pair with in the tavern?
And if you're going to swap Barbarian and Nature, you can move the Archer to the Barbarians and give him initial magic resistance, so that the Barbarian would have 3 skills, like the others.