Covered in ash.

Where is the second Necromancer Castle?

It hasn't been added. Even with a large number of Castles, they can become overpowered due to eliminating reinforcements from other cities. Also, the map's logic, with one additional city on each stage, gives an advantage and encourages a more aggressive playstyle, which means there can't be an even number of neutral cities.

Added 24 minutes later

I played as the Humans with a machine gunner hero and completed the game in 3.5 months. I think there's an obvious excess of units on the outposts. The poor AI couldn't break through them anywhere and quietly awaited its end. If the transitions to the zones weren't so difficult, the game would definitely be more interesting. Also, it's clear that these outposts can be broken through by the Dark faction, which, of course, requires mana. But here's the problem: the author either forgot or deliberately didn't place a single mana well to replenish it, so I had to return to the Castle. By the way, luck and morale boosters would also be helpful. Overall, thank you to the author for the effort.

Yes, unfortunately, the map is very poor for AI. But frankly, I don't see the point in releasing a version for playing against the AI with weakened garrisons, because as soon as the AI gets different Castles, it randomly divides the reinforcements and stops competing at all, so the map still won't be suitable for games with AI.
It doesn't necessarily have to be with the Dark faction, but mana is definitely needed. The lack of mana replenishment is intentional, so that knowledge becomes at least a slightly more useful stat.
As for temporary stat boosts, I just don't really see the logic of how to distribute them