Necropolis or Castle (Which one is stronger?)

That's why I'm saying we need to try our best to eliminate him before he gathers an army.

:D It's easy to say "eliminate him before he gathers an army"... but in reality, defeating a Necropolis hero, especially on large maps with usually... and often superior forces, isn't that simple;) He doesn't move slowly like a computer-controlled hero... he intelligently explores the territory... and correctly distributes "zombies" to potentially transfer forces... and in the later stages of the game, he'll also use Town Portal and Teleport...
If you make a mistake and emerge slowly..."within reach"... then EVERYTHING is lost, the skeletons will strike at your back.... and if it's Galtaran's skeletons, it's even worse:D ...
Lose to a serious opponent like that a couple of times... and then you'll think twice about the power of the Necropolis army on large maps;)