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.
http://en.wikipedia.org/wiki/Arch_of_Constantine