This arch; also known as Arco di Costantino in Italian; is a way of commemorating Constantine I's victory over Maxentius at the Battle of Milvian Bridge.
The Arch of Constantine (Italian: Arco di Costantino) is a triumphal arch in Rome, situated between the Colosseum and the Palatine Hill. It was erected to commemorate Constantine I's victory over Maxentius at the Battle of Milvian Bridge on October 28, 312. Dedicated in 315, it is the latest of the existing triumphal arches in Rome, from which it differs by spolia, the extensive re-use of parts of earlier buildings.
The arch is 21 m high, 25.7 m wide and 7.4 m deep. It has three archways, the central one being 11.5 m high and 6.5 m wide, the lateral archways 7.4 m by 3.4 m each. The lower part of the monument is built of marble blocks, the top (called attic) is brickwork reveted with marble. A staircase formed in the thickness of the arch is entered from a door at some height from the ground, in the end towards the Palatine Hill.
The general design with a main part structured by detached columns and an attic with the main inscription above is modelled after the example of the Arch of Septimius Severus on the Roman Forum. It has been suggested that the lower part of the arch is re-used from an older monument, probably from the times of the emperor Hadrian (Conforto et al., 2001; for a defence of the view that the whole arch was constructed in the 4th century, see Pensabene & Panella).
Arch of ConstantineThe arch spans the Via Triumphalis, the way taken by the emperors when they entered the city in triumph. This route started at the Campus Martius, led through the Circus Maximus and around the Palatine Hill; immediately after the Arch of Constantine, the procession would turn left at the Meta Sudans and march along the Via Sacra to the Forum Romanum and on to the Capitoline Hill, passing both the Arches of Titus and Septimius Severus.
During the Middle Ages, the Arch of Constantine was incorporated into one of the family strongholds of ancient Rome. Works of restoration were first carried out in the 18th century; the last excavations have taken place in the late 1990s, just before the Great Jubilee of 2000.
(Taken from Wikipedia)