I captured the first six castles here quite quickly and easily (although I had to recapture a couple of them), but I deliberately didn't capture the last one, so I could level up as much as possible. By the way, did you know that there's a large land area in this mission, filled with all sorts of goodies?
I didn't know about the land area, but I'll still find it, because I'm going to explore the entire map. If all goes well, one hero with a very strong army will stand guard near the last city and won't let anyone out, while the other will calmly explore the area. Then they'll switch.