Colosseum

Vespasian began it around AD 70 on the site of the artificial lake of Nero's Domus Aurea, and his son Titus opened it in 80 with a hundred days of games. It had some 80 numbered entrances, an awning that shaded the seats and room for about 50,000 spectators, seated by social rank.

For centuries it served as a quarry, and much of its stone ended up in Rome's palaces and churches. Today you walk the first two levels of galleries, and with the Full Experience ticket also the partly rebuilt arena floor and the underground passages where animals and gladiators waited.