The first step was to get a vector map which I obtained from the excellent openvectormaps.com 

You can download extremely detailed maps in vector format there, with clearly labeled elements. I don't know how they do that but hooray nevertheless!

Map after removing all labels and reducing the colors down to the few (!) elements I needed

I then used the 3D rotate function in Illustrator

The graphic was then imported to Photoshop where I was able to give it an extruded aspect (more on that later) and also a thick floor.

Royalty free high-res satellite images are basically impossible to come by, so I knew I would have to use some low-res photograph to use as a texture map on my graphic. The extruded city map I have created before does the trick in achieving a sharp, three-dimensional effect and the relatively low-res photograph gives the colors and busy texture needed. Not a Hollywood high-end CGI job here, but it works.

I then added some terrain and forest elements to the relevant areas, as well as some cracks and strata variations on the floor.

Landmarks added to help locate the projects that the client want to advertise.

Voilà! Final map inserted in a roll-up banner.