Just east of the Roman Forum, stands the colosseum. It was constructed around 70AD and completed in 82AD while Rome was under the rule of Emperor Titus. It was capable of holding around 50,000 people and was used for a number of events, most notably Gladiatorial contests. The correct name for it is the Amphitheatrum Flavium, and the name Colosseum was believed to have been termed due to a colossal statue of the Emperor Nero that was nearby. The statue no longer stands (only the base exists), and was also constantly beheaded to substitute the head of whatever Emperor was in power at the time. The colosseum was used for entertainment purposes until the 6th Century, whereupon it was constantly plundered for its stone and bronze (which was used to build other buildings around Rome), before major damage occurred during the Great Earthquake of 1349. In 1930, Mussolini completely exposed the arena substructure.
Just outside the colosseum also stands the Arch of Constantine, erected in AD 315 to commemorate Constantine's defeat over the Pagan Maxentius, which also ended the persecution of the Christians and allowed Christianity to thrive in Rome.