The Arch of Constantine

This three-bayed arch is the largest Roman triumphal arch to have survived, standing about 21 metres high. The Senate dedicated it in AD 315 to celebrate Constantine's victory over Maxentius at the Battle of the Milvian Bridge in 312, and his first ten years in power.

1An arch made of recycled parts

Look closely: much of its decoration is older than the arch itself. The eight round medallions come from a monument of Hadrian's time, with scenes of hunting and sacrifice; the large rectangular panels in the attic come from a monument to Marcus Aurelius; and the statues of Dacian prisoners crowning the columns date from Trajan's reign. Fourth-century sculptors recarved several of those emperors' heads to give them Constantine's features.

The reliefs that really are from his time, the narrow frieze above the side openings, tell the story of the campaign against Maxentius with stiffer, front-facing figures. Compare them with the medallions and you can see Roman art changing before your eyes.

2A carefully chosen phrase

The central inscription credits the victory to the «inspiration of the divinity», instinctu divinitatis. It is an ambiguous formula that pagans and Christians could each read their own way, at the very moment Constantine was beginning to favour Christianity.

Archaeologists still debate whether the arch was built from scratch for Constantine or reuses the structure of an earlier arch.

3What to look for

• Hadrian's medallions, with boars, bears and lions, above the side openings.
• The captive Dacians up high, hands crossed and wearing Phrygian caps.
• The holes in the inscription: they held bronze letters, now lost.