The weakest castle in Heroes of Might and Magic IV!

The whole problem is the lack of a third-tier unit capable of defeating strong neutral creatures of levels 2-3.

Crusaders first under regeneration, and then under the Guardian Angel (if we're talking about Whirlwinds, it's unfortunately on the fifth level) + illusions to remove retaliation, and everything is great. It's enough for the beginning.
And in general, of course, quickly summon Angels and go ahead.

I prefer to defend the weak, especially when you win with Life or Nature.

Well, you wouldn't call Nature weak. It has access to the combat branch, scouting, destructive magic, buffs, and tactics. Thanks to the archer with the druid/lifewalker and fairies, the start is fast.

Firstly, necromancy was weakened in Equilibrum, and secondly, you need to know how to defeat necromancers and their necromancy. Why weaken a worthy opponent? I don't argue – it's harder to defeat them with necromancy, but it's more interesting and more honorable!

Because it's impossible to defeat a vampire horde. In one battle, 2-3 vampires appear, and after 10-15 battles, you'll have 20-30/30-45 vampires by the third week, maybe even earlier. I haven't played on Whirlwinds for a long time, so I don't remember how it is:)

P.S. Resurrection in Equilibrum needs to be nerfed!!!