Uninvited guests.

This time, by the time the AI first attacked, was the difference not so significant?

Actually, the number of units in the army isn't that important for Necropolis. What matters is what kind of army it is—made up of living or undead creatures. If there were no Necropolis enemies on the map and if they didn't come at the head of the attacking column, there would be almost no difference between 1.0.2 and 2.0.0 for this Castle. It just so happens that the main enemy here for the Necropolis is its own kind. In the version I finished, the computer helped me out a bit. A demon took out a suitable Necromancer. And Lyuska took out the demon with a single stack of vampires (in the field); the rest of the army wasn't needed. If that hadn't happened, the ratio of the Necromancer's army to my hero would have been close to critical. An attempt to fight (in the Castle) without vampires failed, and with vampires, they were targeted first, and the army lost its combat effectiveness. The demon could still be held off in the Castle, and then (the next day) a Warlock came and finished it off.
I started a new 2.0.1 game; the main character left Dexter. The development on neutral creatures turned out well, but when it came to fighting the AI heroes, the difference between Lucretia and Dexter became noticeable. Neither the speed nor the initiative of the zombies comes close to that of vampires, not to mention special skills (blindness/retaliation, flying over obstacles). Dexter lost to the demon outright in the field. He couldn't handle the Necromancer even in the Castle. It was Nadir. The attacker had more mana and troops, and his special abilities were more useful.
I can confirm the report that you can't buy non-standard heroes in the tavern after they run away. So, the promised buyout within 3 days doesn't work.

Added 7 minutes later

By the way, if I build a Blazing Cavern, will the Witch Coven destroy it? Or does it only target the Capitol and the Castle?

Money-making buildings and parts of the Castle will be destroyed. Creature dwellings were not destroyed in my game. If there is no Castle, the Stronghold will be destroyed. I wonder if the magical bindings in the Dwarven Castle will be affected