Measuring some 620 by 513 feet (190 by 155 meters), the Colosseum was the largest amphitheater in the Roman world. Unlike many earlier amphitheaters, which had been dug into hillsides to provide adequate support, the Colosseum was a freestanding structure made of stone and concrete. The distinctive exterior had three stories of arched entrances–a total of around 80–supported by semi-circular columns. Each story contained columns of a different order (or style): At the bottom were columns of the relatively simple Doric order, followed by Ionic and topped by the ornate Corinthian order. Located just near the main entrance to the Colosseum was the Arch of Constantine, built in A.D. 315 in honor of Constantine I’s victory over Maxentius at Pons Milvius.