It's been a while since I played, the hardest ones for me were Rusland and Magiaria I think? You were right in the middle of the map with three aggressive enemies and no choke points, so defending was hard. A lot of raiding parties to deal with. I usually played Renaissance for a relatively easy win but had to switch to Imperial Renaissance. Built lots of Holy Fortresses with gunners. Dragoons were helpful too. The final level (Liguriensis) wasn't so difficult in comparison.

Edit: mixed up WK and WKB, my bad lol. From what I remember I had to retry the last Renaissance mission a few times. The hordes were endless and spawned at several points on the map. Used siege machines a lot and used Artos to repair them.

For some reason I couldn't spawn peasants at the castle in the crusade mission. That one probably wasn't supposed to be hard but it was a slog. The campaign is jank sometimes so that might affect how difficult you perceive each level. I cheesed the final Pagan level by spawning like 10 carracks on the lake in the south and bombarding the patriarch.