Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
Describing Gradus’ visit to Oswin Bretwit (the former Zemblan consul in Paris), Kinbote (in VN’s novel Pale Fire, 1962, Shade’s mad commentator who imagines that he is Charles the Beloved, the last self-exiled king of Zembla) mentions the roofs of Paris: