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a new castle means that the changes are even more significant. At least in Warlords, they didn't touch the parameters of the creatures. So, the players of Stronghold won't like it at all.
quite the opposite, in Warlords there are much more changes compared to Heroes of Might and Magic, changes that directly affect the balance - adding a new city to an existing balance does not affect it in any way. It's the same as if the developers released and added Flax, only now it's not the developers.
Up. the only thing that, in my opinion, seems like a big mistake in Heroes of Might and Magic, is that they deviated from the canonical Heroes concept of a 2-level structure of creatures in a city; after all, 3 levels of building creatures are like a fifth wheel on a car...