The Arch of Constantine
Sitting adjacent to perhaps the most famous icon of Rome, the Colosseum, the Arch of Constantine is an old triumphal monument built in the early 300's. Produced in honor of Constantine's reign, the structure is (according to one Italian scholar), "a real museum of official Roman sculpture, perhaps the richest and most important".
Via Wikimedia
The three arched structure features several corinthian columns in between and above which are reliefs and sculptures of a variety of depictions from Marcus Aurelius' wars as well as sacrificial and hunting based scenes from the reign of Hadrian. These scenes date from much earlier in Roman history and were assembled and plastered together to form one large structure. As any visit to Rome will involve a tourist seeing the Colosseum, I suggest also taking the time to have a quick look at this monument of Roman art. Sources: The Arch of Constantine: Inspired by the Divine By Iain Ferris The Rome Guide: Step by Step through History's Greatest City By Mauro Lucentini