The strongest castle in Heroes of Might and Magic IV.

In my opinion, the strongest are Nature and Haven. Then come Necropolis and Stronghold. Dungeon and Academy are a bit weak.

Of course, this opinion is based on my personal gaming experience.

You're lucky Optimal hasn't seen your statement yet; he's the ultimate authority here.
I won't argue with you, although I believe that Haven, Chaos, Nature, and Necropolis are strong in the early game, while Stronghold and Barbarian are good in the late game. Everyone has their own opinion.