The Arch of Constantine was dedicated on July 25th, 315, to commemorate his victory over his rival co-emperor, Maxentius, at the Battle of the Milvian Bridge, on October 28th, 312.
The Arch of Constantine spans what was once the Triumphal Way. The lengthy inscription explains its purpose:
The Senate and People of Rome dedicated this arch
To the emperor Caesar Flavius Constantinus Maximus
Pius Felix Augustus
For having with his army, by divine inspiration and his own great spirit
Avenged the State with righteous arms
Against both the tyrant and his entire faction.
The arch stands almost 21 m. (69 ft) high and 25.6 m. (92 ft) wide.
More than 50 triumphal arches (a Roman invention) are recorded as having once stood in Rome. The first such arches were set up in 196 BCE, two in the Forum Boarium and one in the Circus Maximus.