[Vynd]

I think that castling can work, but only for some nations and only if you make it a major part of your strategy from the start. You need to plan for the fact that, at least temporarily, you'll be short of gold for building troops and commanders. That'll slow down your research and leave you vulnerable to attack by someone who is spending his money on troops. But you can mitigate this by taking a good combat Pretender and concentrating your early research on getting you research enhancing items. (Or by taking a good research Pretender and concentrating your early research on summoning spells.)

In any event, you shouldn't even think about this sort of strategy if you want to build large numbers of national troops or (to a lesser extent) commanders. Also, as Tuidjy pointed out, it only approaches being practical if you take a cheap castle. And while they will keep your temples from getting burned down, Watchtowers and Mausoleums aren't really much good at stopping determined attacks. Once the fighting kicks into gear you're going to need some sort of method to save your beseiged castles fast, before they fall. Fliers, teleporters, cloud trapezers... something. So your nation needs to be capable of that sort of thing.

In sum, madcastling with Ulm is probably a bad idea, but it could work with, say, Caelum, or Mictlan.