I tried a couple of options, but it didn't work; I don't know why.
He has the same issue in the ToE campaign; it seems the problem is with the model's textures.
Regarding the garrison, I apologize, but the author intended it this way; I've played through it a couple of times myself, and I always cleared the garrison first. Staying on the defensive against two castles when you only have one is a losing strategy. I also tried giving the garrison specifically to the red player, and the mission became much harder. In the original, this garrison holds back both the AI and the player, but when it's given to the AI, it starts rushing from the second week. Giving it to the AI isn't ideal either, because Laszlo, who is even stronger, is waiting further on.
Сценарии:
"Холм мертвецов"
"Священное древо"