Detail from Arch of Constantine, Rome

The Arch of Constantine commemorates Constantine I's victory over the rival Emperor Maxentius at the Battle of the Milvian Bridge on 28 October 312. It was dedicated in 315 and is the last of the triumphal arches in Rome to be built.

Constantine had been raised to the Purple in 306 in York by his father Constantius' soldiers - Constantius had been a member of the Tetrarchy of Emperors devised by Diocletian to manage an Empire threatened on four fronts. Initially not accepted by the other Emperors, Constantine fought his way, first to recognition and then defeating all rivals to become sole Augustus. The core of his power and armed forces were the British and Gallic provinces and Constantive devised a new unit of elite light troops - the Auxilia Palatina, which superseded the legions as the striking force of his new Mobile Field Army.

These new Auxilia Palatina were armed with swords, spears and bows; but unlike the legions were not hampered by heavy armour. They were designed to move quickly and strike ferociously. They were initially recruited from Constantine's core territories of Britain and Gaul and often bore tribal names - although they were in every sense elite Roman regulars.

Constantine's new Arch sits between the Colosseum and the Palatine Hill and spans the Triumphal Way taken by Emperors when they entered the City in Triumph.

The Arch is heavily decorated with parts of older monuments of the "good Emperors" f the past: Trajan, Hadrian and Marcus Aurelius. It is very noticeable that the carving of the former is superior to the contemporary scenes, one of which is shown here.

The reason for re-using the old monuments is probably both symbolic to associate the still perhaps insecure Constantine with the glories of the imperial Roman past. It also shows that classical carving of the past was not something that was easily available.

The scene shown here is from the recent Milvian Bridge campaign, and shows the Siege of Verona which was critical in defeating the Maxentian troops. The Auxilia Palatina are shown advancing to the walls, and one of them is sneaking around the base. A Maxentian soldier is hot down from the walls. It is just possible to discern two small horns on some of the helmets of Constantine's troops, which has led to them being identified as the Cornuti - or horned ones; one of the new elite Auxilia Palatina.

A panel on the other side of the arch shows the actual Battle of the Milvian Bridge on the approach to Rome, with Maxentius' Praetorian Guards drowning in the Tiber.

In depicting a Roman civil war - nothing new in itself of course - on a triumphal arch and showing the deaths of Roman soldiers at the hands of other Roman soldiers, Constantine is doing something new and probably rather tasteless to Roman traditionalists.

Constantine of course did not care for tradition. He demoted the legions in his army, he abolished the Praetorian Guard since they had opposed him, and having seen the sign of the Christian God in the sky at the Milvian Bridge "in hoc sign vincas" he made Chrtistianity the State Religion of the Roman Empire.

Arguably this decision - made gradually - was the decisive moment in the transitions from the Classical to the Medieval World, which is how this Arch should be seen.