But now I'm starting to regret it. I should have entered an empty Castle and tried to survive the siege....
And throughout the game, I never got Darkness... (Although, vampirism plus the staff, which prevented the Imps from moving, would have made it a difficult task).
Overall, I have some doubts about the balance. In my opinion, it should be available to all factions by week 6. There were no significant mistakes or losses (in fact, it was almost the opposite; I was practically given 3 Angels in the first battle without having to buy them before), and I bought all the units (there wouldn't have been enough money on week 5 to buy Devils and Warhorses anyway), so there was practically no chance of winning the siege.
Moreover, it seems that even without vampirism, the siege would be difficult. And since the Underlord needs to hurry, he has to attack, which was also part of the experiment.
If I had arrived on week 5, it might have been a more even match. Although, maybe Darkness would have helped on week 6...
And vampirism... 10 mana is not its real cost