Cities that form their "own little worlds"

Which cities, when you enter them, give you the feeling that you've entered a special world apart, a sense that life is different here from the world you just left? Where do city limits truly become a gateway to something far different beyond? I'm thinking of places like... • Manhattan: cross the rivers and you're in Oz; you have arrived • San Francisco: its own special world, separated from the Bay Area and California as a whole by the wide bay and the roughed divide of the Golden Gate • New Orleans: the mix of cultures is unique and when you leave New Orleans, you have left New Orleans • Pittsburgh: an urban character up there in the Appalachian foothills that speaks of generations living in the same place and an identity of people to city.>