Colosseum

The largest amphitheatre ever built, completed in 80 AD under Emperor Titus and capable of holding 50,000–80,000 spectators for gladiator combat, animal hunts, and naval reenactments. The travertine façade originally rose four storeys with marble seating, retractable awnings (the velarium) operated by sailors, and a complex underground hypogeum where animals and fighters waited beneath the arena floor. Standing inside it is the closest you'll get to ancient Rome — and the most visited monument in Italy for a reason.