The strongest castle in Heroes of Might and Magic IV.

I don't really like the fourth installment of Heroes of Might and Magic... But if I had to choose, it would be either Sylvan or Necropolis... If you level up the summoning magic in the former, you can summon elementals, which is great, especially if you build portals in your cities :) And the latter raises ghosts (or skeletons in a pinch, and vampires as a last resort), which is also great - you can get a pretty good army of ghosts).