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You could say that Necromancers on XL will tear the Castle apart, but you shouldn't forget about early development; I understand that if a battle lasts 3 months, then generally it isn't felt much for the necro, but if Humans get ahead in development, it's unlikely even 2000 thousand skeletons can settle things, due to the difference in hero stats + the quality difference of units.
And overall, strictly speaking, in terms of units, Castle is more interesting than Necros, but it's quite expensive compared to the latter, so we look for money.
And with Necros, we hurriedly collect crystal stones and then clear everything in sight, utopias too :)
As far as I'm concerned, both of these races are not for small maps. The same swamp can easily put them in their place within the 1st month.
If the question is specifically about which growth is cooler, then it's human, but in the long term, in my opinion, castles are equal, i.e., what Humans have that is better is compensated by Necromancer bonuses.
So everything is decided in battle!;)