Which empire built the Colosseum?

The Roman Empire, begun under Vespasian around AD 72 and opened by his son Titus in 80 - and it was a political correction. Nero had seized a huge area of the burnt city after the fire of 64 for a private palace with an artificial lake; the new dynasty drained the lake and put a public amphitheatre on it, handing back what an emperor had taken.

It held perhaps 50,000 people, filled and emptied through 76 numbered entrances with tickets specifying gate, tier and row, and could be cleared in minutes. Beneath the floor was a warren of tunnels, cages and around 28 lifts worked by winches, so animals and scenery appeared through trapdoors. A retractable awning was rigged over the crowd by sailors, since nobody else had the ropework.

It survived by being useful and then by being useful as a quarry. Much of the missing travertine is in Roman churches and palaces, and the pockmarks all over the surviving structure are holes left by people digging out the iron clamps that held the blocks together. It was eventually protected by being reconsecrated as a site of Christian martyrdom - for which, awkwardly, there is no reliable evidence at all.