The strongest castle in Heroes of Might and Magic IV.

Scorpion, did you even read the thread? I think there's a lot of information about this here. I also recommend the thread "The Weakest Faction." There, people initially thought that Haven was the weakest faction. But after a detailed analysis, it turned out to have good potential.
Haven is inherently oriented against Necropolis. Therefore, it has both a sacred spell (and a general sacred spell) and protection against Necropolis. And some units already have it built-in (i.e., you don't need to cast it on them) - monks and knights, I think...