There's always "Three Red Seconds", "Living Castle", and so forth.
I agree, though. I'm hoping for (at least) two levels of castle per nation - one that can be built cheaply in one turn, offers no economic bonus, has only 50 supplies and 25 defense, and maybe improves the quality of the province defense... and the next tier, which offers full bonuses (the official national castle). Having a military fortess choice that gives new troops a +5 or +10XP bonus would be great, too, for people like me that use recruitable troops
The (enchanted) castles in Master of Magic were pretty cool - walls of fire, for example. But as always, there are more neat possibilities than time
