Necropolis or Castle (Which one is stronger?)

I'd like to add my two cents. It's important to consider not just "which castle is stronger," but which army is stronger at the same point in time. And here, if we look at the very beginning of the game, the Castle has an advantage at the start because skeletons and zombies are terrible fighters, and they're also very slow. The Necropolis only starts to "dominate" when liches and vampire lords appear in its army, especially if enough skeletons have been "raised" by that time. Another advantage of the Necropolis is its neutral morale and immunity to mind control spells.
So, the outcome of the war between the Castle and the Necropolis depends primarily on how soon the final battle takes place: the later it is, the harder it is for the Castle.