Personally speaking, I have some experience living in/traveling through an enclave, a citadel, and a ghetto.

I work/have family in College Point, on the Northwestern-most point of Queens. Anyone who travels to College Point (especially someone from Bayside like me) often travels through Whitestone to get there, via 14th Ave, a long street which stretches a long portion of land facing the Long Island Sound. Driving down 14th Avenue, it is evident where upper-class, suburban Whitestone/Malba citadel ends and industrial, middle-class College Point begins (the intersection of 132nd St and 14th Ave, just behind the College Point Shopping Center). It is there where the stretches of mansions come to an immediate halt, and are replaced by  immigrant-owned stores, tighter streets, and duplexes. Even within the neighborhood of College Point, 14th Ave serves as a divide between a citadel and a (arguable) ghetto. Everything on and north of 14th Ave features large mansions and expensive homes similar to those of neighboring Whitestone/Malba. Everything south of 14th Ave (roughly 80-85% of College Point) is rugged in terms of infrastructure, relatively unkempt and congested with traffic.

Peter Marcuse’s claims regarding the nature of the citadel, enclave and ghetto are, in my opinion, spot-on. While these types of neighborhoods, where stark signals are given regarding disparity/inequality of wealth, social structure and infrastructure, are present in every major metropolitan city, they are not as damaging to a society as they may seem. People who populate citadels, enclaves or ghettos arrive there by circumstance, and rarely by chance. People of similar economic/cultural/social association tend to stick together in their own little areas; it is not an issue of intolerance nor unwillingness to assimilate into another culture/social structure. No individuals were coerced into living in areas where they are surrounded by people just like them; upper-class families tend to stick with upper-class families the same way Eastern-European immigrant families prefer to remain near other Eastern-European immigrants, as with Caribbean immigrants, or middle-class South Asians, etc.