The Arch of Constantine is interesting because it has so much history behind it. Emperor Constantine's mother and sister were Christians. Before he engaged Maxentius in the Battle of the Milvian Bridge, he apparently had a vision of a cross in the sky. After winning a decisive battle over his rival, he became sole Emperor and attributed his success to God. His next move was to legalize Christianity. In A.D. 312, the state religion of the entire Western world was proclaimed to be Christianity.
The Arch of Constantine was commissioned in 312 A.D. and completed in 315 A.D. It's purpose was to commemorate Constantine's victory over Maxentius at the Battle of the Milvian Bridge on October 28, 312. The arch was constructed quickly and it is thought that much of the artwork was taken from the ancient buildings of Rome. The arch showed the glory of Constantine's triumphs in battle and in the performance of his civic duties. Also present on the arch are tributes to other Emperors, namely Trajan, Hadrian and Marcus Aurelius. By associating himself with these other "good" emperors, he assured himself a positive place in history.
The Arch itself is approximately 65 feet high and 75 feet wide. The depth of the arch is about 22 feet. There is an attic area at the top of the structure and it is accessed by a doorway in the side of the arch. The arch was constructed out of marble blocks at the base and bricks on the upper attic section. The bricks were covered by marble to complete the monument. The fascinating thing about the Arch of Constantine is that there is so much controversy over how it was built (with pieces from various ancient buildings taken from the Roman Forum) and how many different Emperors are represented.