The idea of building an army by capturing enemy Castles seems doomed – the opponent's main hero makes excellent use of "Portal," moving to the Castle I'm approaching...
I reached the Dungeon in a "record" time – 10 months, but that was more than enough for the opponent to build up unimaginable armies from 6 (or so?) Castles and – somehow – level up their heroes to a level comparable to my main hero...
I'm not a hardcore player, but I have completed difficult maps before; I know ways to defeat a superior opponent, but not when, for unclear reasons, their initiative is always higher than mine, and my stacks are simply wiped out before I even get a turn...
The author has uploaded replays of their battles in the Dungeon. I dealt with Marbas with fewer losses, but the Dark Elf opponents are significantly tougher...
It's a shame – this is the first map in a relatively long time that has really "hooked" me:(
P.S. However, in the final version, the Elf can be quite easily defeated by Raileigh.