There weren't any particular problems with completing this mission. I immediately headed north to gather resources and capture mines. Anastasia defeats everyone, and Lyudmila collects everything (I still can't get used to these Russian names! They just sound awful. "Necromancer Anastasia! It's hell!"). I reached the Inferno Castle and captured it without much difficulty. Anastasia continued to clear the map, while Lyudmila returned to defend the castle. She repelled a couple of attacks. Anastasia entered the nearest portal, reached the castle, defeated the hero, but used up all her mana in the process. She had already visited the Dragon Vein. In short, I didn't conquer this castle right away! =))) In general, the main problem in this mission is the mana issue. Anastasia returned to the main city and headed west, captured another city, and continued to clear the map. They started attacking, but Lyudmila managed to handle it. But only because I didn't immediately convert the Inferno Castle. The Inferno Castle has a good special building that deals damage to each enemy unit during a siege (about 300). This helped quite a bit.
Then, Anastasia decided to capture the Inferno Fort, which had 250 Cave Lords. I had a lot of trouble with this fort! The problem was that I was using the "Subdue" spell on these lords and defeating the Broodmothers with them. And after that, the game would freeze. The solution is to reload. As I understand it, you can't subdue Cave Lords. Or, if you can subdue them, you can't use them to defeat anyone. Otherwise, the game freezes. Well, after that, there were no more problems with the appearance of 20 Spiders and about 100 Liches (and later, even more, of course). There were quite a few enemies in the castles, but I learned how to defeat these hordes with minimal losses.