Aurelian Walls
The Aurelian Walls were built between 270 and 273, by Roman Emperor Aurelian. The 12.5 mile long wall was intended to defend the city of Rome, capital of the Roman Empire, from barbarian attacks. At the time, the city had been grown well beyond the old Servian Wall, built during the republican period, and had been relatively safe during the centuries of Roman expansion. However, by the 3rd century, the new menace of barbarian tribes flooding through the German frontier could not be easily stopped by the Roman Army, with the empire in a heavy crisis.

Construction of the Wall
In order to quickly build the Wall, and for economical and military reasons, many buildings were included in the Wall. Among them were the Amphitheatrum Castrense and the Pyramid of Cestius. A section of an aqueduct, the Aqua Claudia, is also built into the wall near the Porta Maggiore.
While the Aurelian Wall seems not to have been built along the Tiber River, which forms a natural barrier, a salient enclosed part of the Transtiberim (Trastevere) across the river.

Later History of the Wall
In 401, under Flavius Augustus Honorius, the walls and the gates were improved. At this time, the Tomb of Hadrian across the Tiber was incorporated as a fortress in the city defenses. After the fall of the Roman Empire, the popes added additional defenses to the walls to counter firearms.
The Aurelian Wall continued as a significant military defense for the city of Rome until September 20, 1870, when the Bersaglieri of the army of the Kingdom of Italy breached the wall near the Porta Pia.
Due to the need to maintain its defensive ability through the centuries, the Aurelian Wall remains remarkably well preserved today, especially in its southern extent. The Museo delle Mura near the Porta San Sebastiano offers information on its construction and how the defenses operated.
The later Protestant Cemetery is near the walls.
 

From: www.wikipedia.org