The fortifications of the upper town stretch for roughly 1,200 metres and remain largely intact — one of the most complete circuits of medieval walls in northern France. The walls are punctuated by 22 surviving towers, ranging in shape from the standard semicircular to the unusual diamond-shaped Tour aux Engins, which projected the defenders forward and gave them a clear shot down the length of the wall.
The Porte Saint-Jean on the western side is the best-preserved of the main gates: two massive round towers flanking a vaulted passage, with grooves still visible where the portcullis ran. The Porte de Jouy on the southern side is more austere but equally intact.
The walls can be walked along their entire length on the outside, from the public path that traces the line of the original moat. The walk takes about an hour at a relaxed pace and offers continuous views back up at the towers and battlements.