In a major city that has an historical center, the standard list of attractions seldom includes what, in my opinion at least, constitutes the area's most important part: historical residential buildings built a long time ago.
Unlike cathedrals and palaces, these do not seem to be carefully designed. In France (and, specifically, in Paris), these buildings are from two to five stories high and have slanted facades. That's right: the facade is tilted backwards. And they're ubiquitous. All historic residential neighborhoods have them. Just like in the photo below.
You won't find anything like it in any other country (except maybe Belgium).
So, what's the idea?
Is it because in Italy, to pick a place at random, the slap-dash residential construction of yore used bricks and lime render as building materials, while in Paris they favored limestone - is a limestone wall prone to falling over unless you build it like a pyramid? Or does this design feature have something to do with natural light?
Note: sometimes, not always, the ground floor's wall is tilted the other way, like a section of an upended pyramid.
More: These buildings predate the Haussmann's Reconstruction, as attested by the black-and-white photos below. In fact, a lot of them got knocked down to make room for the boulevards. Many remain to this day, however, which is a good thing. They're typical of that city; they're its heart, if you ask me. And they're literally everywhere. Just get off the boulevard - any boulevard - and you'll see them.