In which city is the Colosseum?

Rome, and it was built as a deliberate political gesture. Nero had seized a vast area of the burnt city after the fire of AD 64 for a private palace with an artificial lake; after his fall, the Flavian emperors drained the lake and put a public amphitheatre on it - handing back to the people what an emperor had taken for himself.

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

It stopped being a building and became a quarry. Much of the missing stone 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 saved partly by being reconsecrated as a site of Christian martyrdom - though there is no reliable evidence any Christian was actually killed there.