Which emperor built defensive walls around Rome in the 270s AD? (r. 270-275 AD)

The story behind the answer

Aurelian is correct because he began constructing the Aurelian Walls around Rome during his reign (270–275 AD), amid the Third-Century Crisis and recent barbarian incursions that threatened the city itself. This new 19 km (12-mile) brick-faced circuit, incorporating existing structures like the Castra Praetoria and the Pyramid of Cestius, was Rome’s first major new ring of defenses since the ancient Servian Wall. The project was finished shortly after his death, likely under Emperor Probus.

Memory tip: Aurelian = Aurelian Walls. Don’t confuse him with Hadrian, who built the famous wall in Britain; or Septimius Severus, who campaigned in Britain and repaired frontier walls; or Commodus, who is more known for gladiatorial antics than fortifications.