Walkthrough tips, Discussion of Campaigns [Heroes 5]

Started the add-on. Completed Campaign 1, Missions 1, 2, and 3. Got a bit stuck on the 4th mission. The objective is to destroy the rebels, or rather, one quite strong hero. He's in a castle, and we have 3 castles, but Knights can't be built everywhere, and it seems Monks also can't be built in all castles – but I'm not sure about the latter. Just sitting and accumulating troops isn't very interesting. I've cleared the entire map except for the part where the hero and castle are located (northwest). Maybe there's some trick to it? The difficulty is the second one (I don't know the name).