the city bounds are not revised lightly, but are not ordinarily frozen in perpetuity. They have to be adjusted as the settlement grows or shrinks through time. Even when there is no actual redefinition, the boundary may subsequently lose some of its significance. Overflowing cities, for example, might create such a blanket of settlement outside the walls that their visual impact would be undermined. Conversely, a city like Rome which kept her Classical walls all through the Middle Ages came to occupy a small fraction of the urban land they enclosed. The customs line was a much more realistic circumscription of the abitato, or built-up area.
Spiro Kostof, The City Assembled
* vezi și Stephen Greenblatt: Clinamen. Cum a început Renașterea / mai jos, 38.