The Demons couldn't break through the garrison, neither in the first nor the second attempt. The first time, the orcs deliberately eliminated the guards for some reason. The second time, they wanted to repeat it but were afraid of my hero and retreated.
Gretham broke through the demon outposts, and the orcs, overjoyed, captured the Demon Castle. I watched a cutscene where Knights die while storming the Castle and was surprised. The orcs declared war on the Knights, although it's unclear where Gretham managed to kill even one barbarian hero. Not for long, though; the map finally "came to life." During the five-day war, Rutger and Gretham destroyed all the barbarians.
I didn't understand why the Gremlins with Golems joined (another "long-lasting" quest?). I picked up the Armored troops ("a volunteer squad"), but the Rebel Garrison didn't want to interact with my heroes. I didn't remove the curse and didn't even think about it; Gretham fought perfectly well without magic.
I agree with the previously expressed opinion that the landscape could have been designed more interestingly. The vast territory of the orcs is very monotonous.