The strongest castle in Heroes of Might and Magic IV.

But there's also spell stealing :) and then we look at the strength of the castles without artifacts, and naturally, Haven is the strongest, and it should be strong to maintain balance.

Spell stealing only steals one good spell. This won't help. Angels have high initiative + a hero-general with the Leadership skill. Because of this, angels can move twice: at the end of the turn (because they waited) and at the beginning of the next turn. This gives them a chance to kill the enemy hero. And without a mage, Haven is practically nothing.