You neglect to mention what type of objects you used to create the buildings. If for example you made them with slabs, you can turn on the cover fill option. Similarly you can do this for the roofs or even meshes.
Model View Options (MVO) will allow you to control when these cover fills are seen.
If you created all these buildings with lines, then you made work for yourself. Magic wand slabs, zones etc onto them will give you the coverfill option you want as a byproduct of creating a 3D city.
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"