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I completed the mission in a Jewish style: I reassured the ghost and rushed towards the Castle, intending to reach it quickly, but as I crossed the bridge, the ghost asked again, saying that the assassins were still alive. So, I disregarded any army preparations and launched an assault. I don't know what kind of armies were supposed to be gathered for the assault, but I had 25 dragons, a hundred mummies and liches, 250 vampires, 600 zombies, 52 knights, and 300 spirits. This was more than enough to defeat them. The hero is mediocre; he can't do much besides cloning. If he has the "Cursed Land" skill, they die on their own, and if he has the "Dominate" skill, there are no problems at all.
Added 12 minutes later
I've played all the maps, and I've replayed the game twice from different discs, and I even downloaded a pirated version from a torrent. Everywhere, the same glitch occurs. I suspect that my Arantir has some skills developed in a way that doesn't match the scenario, but this is just a wild guess.