Rome boasts the largest set of city walls in the world. The walls, which remain remarkably well-preserved, were built on the orders of the emperor Aurelian (r. 270-75), who died shortly before they were completed.
Constructed out of brick-faced concrete, the walls are 3.5 m (12 feet) thick and originally ran for 19 km (12 miles). They were punctuated by 380 towers, at intervals of 100 Roman feet (29.6 m), and 18 gates. Originally 8 m (26 ft) high, the height of the walls was later doubled to 16 m (52 ft).
The Aurelian Walls superseded the Servian Wall, which had been built in the early 4th century BCE. The Servian wall was 10 m (33 ft) high in places, 3.6 m (12 ft) wide at its base and 11 km (6.8 miles) long. It is believed to have had 16 main gates.
Only a few short stretches of the Servian wall, which was built out of large blocks of tuff quarried from the Grotta Oscura quarry near Veii, remain.