All castles in Heroes 3.

Yes, Barbarians might have better attack, but their defense is terrible compared to the Fortress.

Well, sorry, but that's how it's designed... Barbarians get attack, Fortress gets defense... It's actually quite convenient for gameplay: some people like to rush and overwhelm their opponents, while others prefer to wait and slowly but surely defeat the enemy... IMHO, that's how it should be! Each faction has its own strengths...
P.S. I've never liked playing as Dungeon; I don't think it's a very strong faction... And Darklings aren't among my favorite creatures...(THIS IS MY PERSONAL OPINION... and please don't try to convince me otherwise!)