The idea of destroying castles down to their foundations and then rebuilding them over and over again, spending resources on it, is not viable. Why build it if it's practically impossible to hold? You can't move your main hero to defend each castle, and a secondary hero in a half-built castle can't withstand a siege...

Nonsense! - in my opinion, of course. In reality, we get a situation where losing a castle is an unacceptable luxury, and mines that don't produce gold remain in demand throughout the game, not just in the first 3 weeks. This encourages controlling territory, developing secondary heroes, and playing a full game throughout the match (usually, after the main buildings are built, the need for resources disappears, and so does part of the gameplay - capturing and controlling mines, city building). And there's simply no point in fully rebuilding secondary castles - it's enough to partially rebuild a couple of castles and hold them with secondary heroes (with the right distribution of forces, it will work). In this case, you can go for capturing a Necropolis and rebuilding and holding it with a secondary hero focused on defense (I invested in Mentoring and created three secondary heroes), or you can capture some corner castle and push back the enemy, having a couple of protected cities behind you. I agree with the comments about the unnecessary quests (this is Multiplayer, not a single-player game), as well as with the comment that there is no conversion of dwellings for the dwarves (the conversion happened for the Necropolis, but I don't remember about the others)