Colosseum

Emperor Vespasian began the Colosseum around 72 AD, funded partly by the plunder from the sack of Jerusalem, and his son Titus opened it in 80 AD with 100 days of inaugural games. At full capacity it held somewhere between 50,000 and 80,000 spectators across four tiers, entering and exiting through 80 numbered arches in a flow system so efficient that modern stadium architects still study it. Gladiatorial contests, staged hunts, public executions and even mock naval battles (achieved by flooding the arena) took place here for roughly four centuries before the games were formally ended in the early 6th century.

What you see today is a genuine survivor rather than a full monument — earthquakes, centuries of stone-robbing for other Roman buildings, and general decay stripped away the marble facing and much of the upper structure, leaving the raw travertine, brick and concrete skeleton exposed. That skeleton is still enough to make it one of the most instantly recognisable buildings on the planet, and the numbers back that recognition up: it draws more visitors annually than almost any other single monument in the world.