The weakest castle in Heroes of Might and Magic IV!

When deciding which Castle is the weakest, you need to specify the map size. Chaos is quite weak at the beginning, and 1-2 Chaos Dragons won't win against upgraded Spiritual Armor, Defender, Bless, and Prayer.

Why do you think so? Even at the initial stages, it can win if you know how to play...

Added 7 minutes later

Thanks to the upgrade of mass buffs, Life compensates for what it lacks. Only Death and Order have mass dispel, and even then, only if you find it in the Guild.

Order and Death are different cases. Order is a race focused on magic. It's impossible to play Death at high difficulty levels without resurrection. Nature provides great help with summoning and the portal, where you can get Water Elementals or Manticores. Might provides a large bonus, and building a menagerie greatly simplifies the game. Chaos has strong units at high levels and the strongest magic, mainly focused on attack. Life has mass buffs, which are necessary at high difficulty levels; you need 2 Priest heroes.