The Servian Wall was a defensive barrier constructed around the city of Rome in the early 4th century BC. The wall was 3.6 m thick, 11 km long, and had more than a dozen gates.
The Servian Wall is named after the sixth Roman King, Servius Tullius. Although its outline may go back to the 6th century BC, the currently extant walls were probably built during the later Roman Republic, possibly as a way to prevent a repeat of the sack of Rome during Battle of the Allia by the Gauls of Brennus. Due to the ease with which the Gauls entered the city, it is conjectured that at some time previous to this, Rome had been forced by its Etruscan rulers to dismantle any significant prior defences.
The Servian Wall was formidable enough to repel Hannibal during the Second Punic War. Hannibal famously invaded Italy across the Alps with elephants, and had crushed several Roman armies in the early stages of the war. However, the walls were never put to the test as Hannibal only once, in 211 BC, brought his Carthaginian army to Rome as part of a feint to draw the Roman army from Capua. When it was clear that this had failed he turned away.
Sections of the Servian Wall are still visible in various locations around Rome.