The strongest castle in Heroes of Might and Magic IV.

Unfortunately, I believe NECROMANCERS are the strongest, based on my personal experience.

LeSHiJ-MaX, I'm not insisting that I'll change your mind, but... As a fan of Sylvan, you should understand that against Death, this is one of the worthy opponents.
Vampires will be defeated by water elementals (you can, of course, use others, but I think this is the most worthy). Moreover, it has a cold attack, even when it's protected, for example, by vampires, but they won't get a charge from it to restore their lives.
Sylvan has a powerful WEAPON - the Necromancy Amulet. It will prevent Death from raising the dead.
This is the most important thing. Therefore