here's another thought.
um.. why have hard pop caps? wouldn't a player (ie.. region lord) imposed cap be more interesting? (ie.. a player can figure out how much food region produces.. what pop cap it can support and then increase/lower the cap to dictate the food surplus/deficit and then do whatever about the surplus/deficit)
for that matter.. don't define rurals/cities/towns.. just have base terrain type (plains, whatever civ style tile) limiting resource production, then let the players dictate the pop. very crowded region with a food deficit would be equivalent to a city, for example.
have a cap on the number of pop available to dedicate to farming or whatever food. (to simulate more land used for living, less land for farming)
by removing towns/cities/rural (because those are description relative to pop rather than terrain), you can allow people to build castles anywhere with the pop to sustain it. and allow every region to be capital, etc.. if the players want it.
and that you don't have to besiege castles just to go march past them... surely the whole point of most castles is not that you block the road with them literally and physically.. but that if you leave one in enemy hands behind you, the enemy can base around it and hit you from behind.
a long term goal perhaps?
