The way I'm going about mine is a modular approach. Basically I went to a home improvement store and got myself a sold sheet of 3/4"
MDF and got it cut into (roughly) 12"x12" pieces. Now I'm spraying them with dark gray textured spraypaint for that "urban" look. Initially I was going to secure all the buildings (scratch built from foamboard) to the pieces but I decided against it; now they're just going to sit on top. The
MDF bits now form a solid table base which I can put on my already existing gaming table.
City boards are often just a mess of buildings with the occasional road. Don't be afraid to try something different. Even in filthy, run down cities there are open spaces where green has grown back through. There are parking lots and street corners with considerable amounts of space. Roads don't have to be straight or intersection shaped, you can do things like traffic circles. Railways often run through cities.
Check out Terra Genesis:
http://terragenesis.co.uk/ M.