Necropolis or Castle (Which one is stronger?)

I'm not disrupting the balance. On the contrary, the undead are stronger with mid-level units (vampires and liches), but weaker with higher-level units (dragons). Due to their numbers, skeletons are stronger than spearmen, but archers are better than zombies. So, the balance is quite well maintained. It all depends on how you use the strengths and weaknesses of each faction.