Thanks Burrman, I know Geneva is kinda flat (live there) but there's a lot of streets that have some slope.

The problem is when you look at the cutaway is that there's two main entrances at two different levels (number 1 and number 2)

Also there's some streets at number 3 that are on a different level than streets at number 4

https://dl.dropbox.com/u/44982094/Capture%20d%E2%80%99%C3%A9cran%202013-03-28%20%C3%A0%2009.42.43.PNG

So I can't really make everything "flat" it need some kind of level difference.

I'm calling a guy to get topology in polygons then I guess I'll just adjust it to fit the buildings inside of Cinema 4D.

Thanks for your help :)