The most challenging campaign.

For me, difficult campaigns are those with static elements. That is, those with a large map and many castles that you capture, build up, and so on. In Agrael, there were difficulties with Gilraen (the unpatched 1.0 version), but it was still passable – with a bit of cunning. And the next one is tedious – you run around for a long time, and the city with the port is very strong. In Markel, there's the mission with the curses. It glitched badly for me. The cities weren't being cursed, and even 20 dragons didn't produce the desired result. I had to resort to cheating. But that was only after I had defeated everyone else. And the last one is simply scary because of the unexpected appearance of a second army – you don't expect it right away. I only replayed the final battle twice, and left everything else as it was. I summoned a bunch of elementals with Markel and went for it. Angels are, of course, good, but they can't shoot. In Railag, it's a bit difficult with seven castles (although, to be honest, it's with five, since two can be captured immediately by everyone).
It's just that everywhere you need to hold out until a critical point, while the growth of power is not in your favor, and then (you immediately feel this moment) the initiative is yours, and you can frolic on the map as much as your heart (or the scenario) desires. It's been like this many times – you balance, you survive, and then you strike successfully, and that's it, game over