The Colosseum is an oval amphitheater in the center of the city of Rome, Italy, just east of the Roman Forum. It is the largest ancient amphitheater ever built, and is still the largest standing amphitheater in the world today, despite its age.
Construction began under the emperor Vespasian (r. 69–79 AD) in 72 and was completed in 80 AD under his successor and heir, Titus. Further modifications were made during the reign of Domitian. The three emperors that were patrons of the work are known as the Flavian dynasty, and the amphitheater was named the Flavian Amphitheatre by later classicists and archaeologists for its association with their family name (Flavius).
The Colosseum is built of travertine limestone, tuff (volcanic rock), and brick-faced concrete. It could hold an estimated 50,000 to 80,000 spectators at various points in its history, having an average audience of some 65,000; it was used for gladiatorial contests and public spectacles including animal hunts, executions, re-enactments of famous battles, and dramas based on Roman mythology, and briefly mock sea battles. The building ceased to be used for entertainment in the early medieval era. It was later reused for such purposes as housing, workshops, and quarters for a religious order, a fortress, a quarry, and a Christian shrine.
Unlike Roman theatres that were built into hillsides, the Colosseum is an entirely free-standing structure. It derives its basic exterior and interior architecture from that of two theatres back to back. It is elliptical in plan and is 189 meters long, and 156 meters wide, with a base area of 24,000 square meters. The height of the outer wall is 48 meters. The perimeter originally measured 545 meters. The central arena is an oval 87 m long and 55 m wide, surrounded by a wall 5 m high, above which rose tiers of seating.
The outer wall is estimated to have required over 100,000 cubic meters of travertine stone which were set without mortar; they were held together by 300 tons of iron clamps. However, it has suffered extensive damage over the centuries, with large segments having collapsed following earthquakes. The north side of the perimeter wall is still standing; the distinctive triangular brick wedges at each end are modern additions, having been constructed in the early 19th century to shore up the wall. The remainder of the present-day exterior of the Colosseum is in fact the original interior wall.
Although substantially ruined by earthquakes and stone robbers taking spolia, the Colosseum is still an iconic symbol of Imperial Rome and was listed as one of the New 7 Wonders of the World. It is one of Rome’s most popular tourist attractions and also has links to the Roman Catholic Church, as each Good Friday the Pope leads a torchlit “Way of the Cross” procession that starts in the area around the Colosseum. The Colosseum is depicted on the Italian version of the five-cent euro coin.
According to Wikipedia