More formally known as the Flavian Amphitheater, the Colosseum was built between 72 and 80 AD. The Colosseum could hold, it is estimated, between 50,000 and 80,000 spectators, having an average audience of some 65,000; it was used for gladiatorial contests and public spectacles such as mock sea battles (for only a short time as the hypogeum was soon filled in with mechanisms to support the other activities), animal hunts, executions, re-enactments of famous battles, and dramas based on Classical mythology. The building ceased to be used for entertainment in the early medieval era. It was later reused for such purposes as housing, workshops, quarters for a religious order, a fortress, a quarry, and a Christian shrine.
Read more about the Colosseum (link opens in a new window).
Despite what many history books say, slaves were not used to build things like the Colosseum- these were built with skilled workers whose job it was to build buildings. Slaves were used for lower jobs, like maids and kitchen workers. It would be like a construction project today- people are paid to do the different stages of working and that's their job.