Can't you make an option that if an alliance or a single player (which won't happen probably) owns a whole state such as South Africa or Ireland or something that they can put up a primary defence around it and other players need to break through that first before getting to the city they after. So then you don't need to focus on defending each city but mainly on your primary defence... Just a thought...

Been suggested many times.

A variation,but essentially the same:
http://www.gewar.net/forums/showthread.php?t=795
posted less than 3 days ago

actually, this is different, and i like the idea.

i've always been for forming some sort of "front line"