The problem is: The cities are 1 tile on the map. And as such MUST be represented in a unique battlefield. It is possible to alter the layouts of the capitals modifying the streets layout and such, and even add a few distinctive buildings for capitals, but the cities will still be represented by a single square on the map, and thus cant be shown in their full beauty.

Alternatively, what could be done is create a battlefield that would BE a city and thus having an urban warfare map, but then the "assault the walls" factor would be lost as armies would be placed straight "inside" the city.

I dont know if I explained well enough, but heh...