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Well, Necropolis is definitely a strong Castle with its own unique characteristics.
The main schools of magic that are beneficial for Necropolis heroes to use are:
Fire, Air, and Earth. The Castle almost always has the spells "Raise Dead" (although there have been a few cases where it didn't work), "Portal to Town," and "Anti-Magic."
It's desirable to acquire Fire magic spells such as "Berserk" and "Blindness." These spells don't affect the Necropolis warriors themselves.
Air magic is optional.
On large maps, I think you should aim to acquire an artifact like "Cloak of the Dead King" (Necromancer's Boots + Mortuary Pendant + Vampire's Robe).
I believe it's better to develop a Necromancer rather than a Death Knight. A mage is more versatile.
It's extremely important for Necropolis to capture the "Medal of Vulnerability" artifact before your opponents do.
A serious drawback of Necropolis is its slow development, which can later become a cause for defeat.
P.S. I just downloaded a map that is quite challenging to play. However, Necropolis is one of the main Castles with which (in my opinion) you can win.