The World's Longest Set of City Walls

A section of the Aurelian Walls

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Rome boasts the longest set of city walls in the world, most of which are still standing. The walls 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, they are 3.5 m (12 feet) thick and originally ran for 19 km (12 miles). The walls 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). 
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The Aurelian Walls, which remain remarkably well-preserved, superseded the Servian Wall, which had been built in the early 4th century BCE.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. 

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A section of the Aurelian Walls on the south side of the city

Porta Asinaria

Porta San Sebastiano

A stretch of the Servian Wall at Termini Railway Station