Necropolis or Castle (Which one is stronger?)

In my opinion, each faction is good for its own purposes. Necropolis is good for skeletons, meaning for prolonged wars and larger maps. They also perform well in the mid-game with Black Knights and Vampire Lords. Fortress is good for rushing, and for the mid-game; Archangels are fast. There's nothing to overthink here; you can succeed with any faction, the balance is there...