The Flavian Amphitheater was built by Emperor Titus Flavius Vespasian starting in 70 AD, but it is universally known as the Colosseum because it was built on the site of a lake that was part of the gardens of Nero's Domus Aurea, where a colossal statue of him portrayed as the Sun God stood.
All kinds of spectacles were held in the Amphitheater. Many were bloody, such as those with gladiators, but these followed very specific rules, choosing the fighters according to their armament, so that the fight was on equal terms.
Then there were venationes, or the hunting and killing of exotic animals. Augustus boasted of having financed these kinds of spectacles, in which as many as 3,500 wild beasts, specially brought from Africa, were slaughtered.
Beneath the Colosseum arena, the subterranean areas, which are rarely open to the public, still exist. These contained quarters for gladiators and slaves, as well as storage areas for sand and other materials.
They were essentially the subterranean wings of a theater: shows were prepared there, scene changes were prepared, and gladiators awaited their turn to fight.
Wild animals were also kept in the subterranean areas; traces of the tunnel-like cages through which they were moved can still be seen, just like in modern circus shows.
There were special hoists powered by winches and counterweights: they were used to lift wild animals or gladiators up to the arena level, making them appear unexpectedly, just as seen in Ridley Scott's famous film "Gladiator."
The Colosseum was also used for historical reenactments of the most famous battles of antiquity and for theatrical performances (often linked to religious cults), including Greek tragedies or Latin comedies.
For centuries, the Colosseum was a convenient quarry for building materials, as the enormous travertine blocks (visible in the foundations). Even the metal clamps that held them together were removed, along with all precious marbles and sculptures.
The mass persecutions of Christians for which the Colosseum is sadly famous began around the middle of the 3rd century AD.
In the late Empire, the emperors spent enormous sums on circus shows, which took place almost daily: a way of gaining popular favor, and distract people with the classic «panem et circenses» (bread and circuses).