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Frankly, I don't consider this an undeniable advantage that elevates Necropolis above all other Castles. A couple of external dwellings, and the number of centaurs increases several times.
And are you 100% sure that you will have such spells? :) Well, you're an optimist.
1. Necromancy is much more effective than external dwellings, and on some maps, they don't even exist. And if they do exist, they're not for your castle. :)
2. You just need to build the Mage Guild to the end, and then such spells will definitely appear. ;)
3. Besides Necropolis, you can also highlight Humans. A balanced race, and therefore very strong.