One of the three triumphal arches in Rome and the largest surviving one is located near the Colosseum. It was installed there in 315 AD in order to commemorate the victory of Emperor Constantine in the Battle of Milvian Bridge in 312 AD.
While the arch itself was made for this purpose, not all of its parts are original. The statues decorating it were originally placed at Trajan's Forum. Also the marble medallions that can be found on the arch were taken from some of the monuments dedicated to Emperor Hadrian.
There are no entrance fees and the arch is open to public all times of the day. It can also be spotted from the Colosseum.