I recently started playing Heroes 3 (I played it about 6-7 years ago).
I read the entire thread about this faction, but I must have missed something:
It is often mentioned that on random maps, hives (neutral) are placed in swampy areas, and in them, they recruit wyverns...
Aren't snakes recruited in hives?
I play on 160%, and I usually place a hydra lair towards the end of the week, only if I'm very lucky, and even then, I can't always build a stronghold...
I enjoy playing as Fortress, but I don't consider swampy terrain to be a plus. It's difficult to recruit troops of other factions at the beginning of the game (they can only be recruited when hiring heroes in the tavern), and if the opponent does run towards the castle, they will go along the road and not deviate to mines, etc.