The Arch of Constantine stands beside the Colosseum and was erected soon after AD 315 to mark Constantine's victory over Maxentius at the Battle of the Milvian Bridge. Its reliefs reuse sculpture from earlier imperial monuments, making it both a triumphal arch and a visible summary of Rome's habit of building on its own past.
It is easy to see while visiting the Colosseum, Palatine Hill, or Roman Forum, but it rewards a slower look at the mix of carvings from different eras.