I don't know anything about Constantinople, but as far as Rome goes...what ethnic quarters? There's a small amount of evidence that at least at one point some vici were dominated by particular ethnic groups, but this evidence is very early and if it was the case that, for example, the Vicus Africus that Varro mentions was, as he says, populated by refugees from the Punic Wars then by the late Republic whatever ethnic character the area had had totally dispersed. And vici of course weren't real neighborhoods, they were more like administrative districts--Greer thought of the vici as having very strictly-defined and segregated cultural and economic distinctions between them, but this has generally been abandoned and vici are usually though of simply as administrative sub-units. The only known ethnic neighborhood in the city is, according to Lott's excellent book on Augustan vici, the small Jewish neighborhood at Trastevere. As far as similar people clustering in locations went, we don't find evidence for ethnic divisions as being significant. This is also seen at other Italian cities--at Pompeii there is no material evidence for the segregated ethnic communities, even though we know that there were Greeks, Romans, Oscans, and plenty of other Italians living in the city. What evidence we find for such groupings is in trade--tradesmen tended to cluster up in particular places, usually obvious in their locations. So we're told that book-sellers set up near the forum (a good place for it) and wine traders clustered near the docks (duh). This shouldn't be taken too far, though. Just because there was a community of butchers at the piscina publica (the lanii piscinenses) doesn't mean there were no butchers anywhere else or that butchers were found in particular groups--this seems not to be the case at all, there's no material or textual evidence for it. The ancient city was highly diverse, a true metropolis, but this diversity is generally characterized by the mixing of populations (ethnic, socio-economic, etc.) rather than their segregation. One wonders how, realistically, ethnic populations could reasonably keep themselves separated in the city. Rome's population was constantly shifting as immigrants flocked into the city, slaves were freed, day-laborers moved around searching for work, the urban poor died off from disease, and seasonal migrants entered from the rural and suburban districts to search for work. In any given district the number of permanent residents would have been a relatively small minority, and even among those there would have been some considerable mixing, particularly since many of the urban dwellers who could afford to have permanent housing would have been skilled workers, often freedmen of all different kinds of ethnic backgrounds