Hooray! I made it!
Although I had to start over...
I reached the last castle in almost 5 months. I would have gotten there sooner, but an Inquisitor ran away from me and captured two Fortresses and a Castle. I had to recapture them. I didn't lose any troops, so there was no one to resurrect. Thanks to someone's advice, I didn't rebuild the Castle into a Necropolis, or rather, I didn't do it right away. Thank you for suggesting that there is a market with artifacts. I bought myself a cloak with +10 to magic resistance.)))
The threat in the Castle was high, but I didn't use scrolls - I won without any problems. I didn't lose any Ghouls at all, and there were only a few Specters and Skeletons. I blinded the archers for 2 turns and shot at the Shimmers, there were 42 of them. Griffins flew out, I dealt with them, then Guardians came out, and so on.
I had 260 Ghouls, about 400 Specters, and as many Skeletons.
I didn't clear the map, and I forgot to go to the Arena (I just remembered).
I need to find the save file and go to the Arena.)
I upgraded the Hero to Dark Magic level 2, experience, physical defense 1, Agony 1, Drain and Weaken level 1 and 2.