Under the control of a skilled player, any castle will be strong. I don't think there are any weak factions in this game; they are all equal, each with its own strengths and weaknesses, which either save or complicate the situation at a certain point. I wouldn't dare to call any city the worst, because they are all unique and strong in their own way, and you can develop them all successfully. I've tried playing with each faction, and it turned out quite well, so friendship won. At least, that's how it was for me. :)