-Download Citygen plugin from [link]
and copy it into your cinema 4d plugin folder.
-Generate city like in tutorial (link to page) and copy paste a little on your floor object.
-Make some of your buildings like I did on other pictures.
Take care when you play with polygons. Rendering is slow and long process. Test picture first with render region. I hope this will help at least to start your own city. Tutorial is made from my work in progress collab with everyone who send me a building or some techy urban thingie.
Experiment and ask if you stuck somewhere.