What Roman wall was built by Emperor Aurelian in the 3rd century to defend Rome?

The Aurelian Walls were built by Emperor Aurelian to defend Rome in the 3rd century.

Construction began in AD 271, during the Crisis of the Third Century, after Germanic invasions exposed Rome's vulnerability. The circuit enclosed the expanded city beyond the older Servian Wall and incorporated existing structures, including parts of the Praetorian Camp and the Pyramid of Cestius.

The walls originally ran for about 19 kilometres and were built mainly of brick-faced concrete, with towers and fortified gates. Although Aurelian died before the project was fully completed, construction finished within a few years. Later emperors strengthened and raised sections of the defenses.

A common mix-up is with Hadrian's Wall, which marked Rome's frontier in northern Britain, or the Servian Wall, the much older Republican fortification around Rome. The Aurelian Walls remained important until the capture of Rome in 1870 and substantial sections still stand today.