Thanks a lot , guys!
I was very pleased by the ground. In these times, Paris was a giant dump. The old roman sewers only covered a small part of the city. So the paved and cobblestone streets and alleys were covered by mud (not to say worse...)
I was inpired by these photos taken in the city of Bourges during the shooting of a new version of the 3 musketeers (a real piece of shit!) some years ago


And I really like this kind of muddy ground so that's what I tried to do with my streets ( pieces of cobblestone and a mix of sand, plaster and pva glue. )
If you like Medieval city, Bourges is a very nice one with splendid old building and renaissance Palaces and a great cathedral (nearly as old as Notre-Dame in Paris). You can check this website with some nice pictures.
http://bourges18.free.fr/index.phpCheers,
Olivier