Keep in mind that the next Necromancers will be even tougher!
Let me clarify some points about the first siege. By this point, it won't be possible to build a Castle, but a Stronghold should be. The Necromancer doesn't use Phantom, that's what the next ones do. He has a powerful tent, and it wasn't possible to break through it with griffins. But it can be distracted on individual armored units as if it were a plague.
I decided to start all over again. I defended the outpost very well, and the Necromancer arrived after a week. But he came with 4 more Black Dragons, and despite the heroic defense of the undead, the dragons and the dead defeated me. I tried to abandon the Castle and make a circle. When I retook the Castle, another tough one arrived. This is not an option. I think the main thing is to do well in the first outpost defense. But I'll try again.
Question for the author: According to the quest, Laslo or Alaric's death is not allowed. But in reality, the game continues, although they cannot be hired again.