By the third century AD, the boundaries of Rome had grown far beyond the area enclosed by the old
Servian Wall, built during the
Republican period in the late 4th century BC. Rome had remained unfortified during the subsequent centuries of expansion and consolidation due to lack of hostile threats against the city. The citizens of Rome took great pride in knowing that Rome required no fortifications because of the stability brought by the
Pax Romana and the protection of the
Roman army. However, the need for updated defences became acute during the
crisis of the Third Century, when various tribes moved through the Germanic frontier and the Roman army struggled to stop them. In 270, groups of
Juthungi and
Vandals invaded northern
Italy, inflicting a
severe defeat on the Romans at Placentia (modern
Piacenza) before eventually being driven back. Further trouble broke out in Rome itself in the summer of 271, when the mint workers
rose in rebellion. Several thousand people died in the fierce fighting that resulted. Aurelian's construction of the walls as an emergency measure was a reaction to the invasion of 270; the historian
Aurelius Victor states explicitly that the project aimed to alleviate the city's vulnerability. It may also have been intended to send a political signal as a statement that Aurelian trusted that the people of Rome would remain loyal, as well as serving as a public declaration of the emperor's firm hold on power. The construction of the walls was by far the largest building project that had taken place in Rome for many decades, and their construction was a concrete statement of the continued strength of Rome. Parts of the wall were doubled in height by
Maxentius in the period 306–312 AD, who also improved the watch-towers. In 401, under
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. ==Later use==