Uninvited guests.

I'm far from being a newbie in Heroes.
P.S. For example, it's quite easy for me to play as a Knight.

If you're not a newbie in this, then what prevents you from reading the thread about this map first? You wouldn't even have the questions – whether it's possible to win the map as a Knight or not, and with which factions. Secondly, if it's easy to play as a Knight, but it's completely impossible to play as a Hero – something doesn't add up here. The difference between difficulty levels isn't that significant.

So, Subjugate can't even drop in the Dungeon castle, and the chance for Berserk is small (if it exists at all), while the Dungeon has Chaos and Summoning.

Not entirely fair. In the Magic Tower, random spells drop. There is a certain increased probability of a spell dropping in a specific castle. On this map, I had a Dungeon castle where, out of all the "chaos," only Stone Skin and Fire Wall dropped, and finally, on the 5th level – a curse. I restarted.