The strongest castle in Heroes of Might and Magic IV.

I don't know what you're talking about regarding the strength of the Sylvan faction. But I have repeatedly defeated it with Haven, even when my army was three times smaller. I'm not saying that Sylvan is weak. But it still yields to the strength of Haven. As do practically all other factions. Because in order to maintain order, one must possess strength. And this faction has gathered the strong sides of both good and evil.
As for exorcism, the magic of Haven has almost the same spell – dispel magic – and it also costs 2 mana.