From the Conflux, any Castle will take a beating, just like any other Castle will take a beating from any other. The balance is there, even in Africa. In terms of magic, it's definitely the strongest; Dungeon and Tower can't keep up. Although the Mages in these Castles are very good. Everyone praises Solmyr, but I think Deemer is a little more powerful, especially if Alamar is his assistant. But that's a topic for another discussion...
Out of this trio, they're all good; it's impossible to say definitively which is the best. I've played as them and against them, and of course, I've beaten everyone, starting with the easiest – Conflux, then the rest.