all creatures can break down a wall. Absolutely all creatures. That is, your workers can go and try to break down the gates with their little fists. In previous games, this wasn't possible. Creatures would stand and wait to be killed by archers, or until the gates broke down on their own.
In Heroes of Might and Magic IV, gates could only be broken down manually. Catapults were present as units, but not as machines for destroying walls.