Throughout the history of Rome, the urban limits of the city were considered to be the area within the city walls. Originally, these consisted of the Servian Wall, which was built twelve years after the Gaulish sack of the city in 390 BC. This contained most of the Esquiline and Caelian hills, as well as the whole of the other five. Rome outgrew the Servian Wall, but no more walls were constructed until almost 700 years later, when, in 270 AD, Emperor Aurelian began building the Aurelian Walls. These were almost 19 kilometres (12 mi) long, and were still the walls the troops of the Kingdom of Italy had to breach to enter the city in 1870. Modern Romans frequently consider the city’s urban area to be delimited by its ring-road, the Grande Raccordo Anulare, which circles the city centre at a distance of about 10 km.
The Commune of Rome, however, covers considerably more territory and extends to the sea at Ostia, the largest town in Italy that is not a commune in its own right. The Commune covers an area roughly three times the total area within the Raccordo and is comparable in area to the entire provinces of Milan and Naples, and to an area six times the size of the territory of these cities. It also includes considerable areas of abandoned marsh land which is suitable neither for agriculture nor for urban development.
As a consequence, the density of the Commune is not that high, the communal territory being divided between highly urbanised areas and areas designated as parks, nature reserves, and for agricultural use. The Province of Rome is the largest by area in Italy. At 5,352 square kilometres (2,066 sq mi), its dimensions are comparable to the region of Liguria.