Is Conflux a broken castle?

Conflux is a good castle, but it's definitely not overpowered (it might be unbalanced, but IMHO, that's not the case). I've played both with and against it, and I think REgulus is right: you just need to know how to play with different castles. I've used Conflux to defeat Rampart, Necropolis, and Haven. Playing as Rampart or Necropolis, I've also defeated Conflux. The fact that the castle is strong at the beginning is offset by its weakness in the late game. You also need to find the Grail and the necessary artifacts, especially if you're playing on large maps.

In the end, Conflux is strong in some areas and weak in others. That's what's required for good gameplay. And that's all there is to it...