hm you all forget the fantasy part of dominions ihmo . in middle ages building forts was really taking a long time and if a fort was badly damaged i agree the rebuilding needs almost the same time as building a new one .
but that's in the middle ages .
while in dominions you have powerful mages / your pretender god . they are so powerful it should be an easy task for them to let their minions like demons .... build / repair a castle in no time . you even have some spells for this .
in almost every major rpg great wizards let devils/demons build their great wizard towers during one day
but pvk's idea is nice . but you could implement additional spells then , best one earth magic and one blood magic spell which lets you repair your fortress via ritual . so you have the choice e.g. pay half of the fort's building costs or pay e.g. 5 earthgems or 10 bloodslaves to let it be repaired .
some nations like ulm who are good castle builders should get a discount and only pay 25% .
ermor perhaps should pay nothing cause they have upkeepfree population . cause the gold you pay are mainly the wages for your workers who build the castle while you get the stones ... for free ( at least in the middle ages ) .