A monument to raw, merciless power, the Colosseum (Colosseo) is the most thrilling of Rome's ancient sights. It's not just the amazing completeness of the place, or its size, but the sense of violent history that resonates: it was here that gladiators met in mortal combat and condemned prisoners fought off wild beasts in front of baying, bloodthirsty crowds. Two thousand years later it's Italy's top tourist attraction, pulling in between 16,000 and 19,000 people on an average day.

Built by the emperor Vespasian (r AD 69–79) in the grounds of Nero's palatial Domus Aurea, the Colosseum was inaugurated in AD 80. To mark the occasion, Vespasian's son and successor Titus (r 79–81) held games that lasted 100 days and nights, during which some 5000 animals were slaughtered. Trajan (r 98–117) later topped this, holding a marathon 117-day killing spree involving 9000 gladiators and 10,000 animals.

Originally known as the Flavian Amphitheatre, the 50,000-capacity stadium may have been ancient Rome's most fearful arena, but it wasn't the biggest – the Circo Massimo could hold up to 200,000 people. The name Colosseum, when introduced in medieval times, was not a reference to its size but to the Colosso di Nerone, a giant statue of Nero that stood nearby.

The outer walls have three levels of arches, articulated by columns topped by capitals of the Ionic (at the bottom), Doric and Corinthian (at the top) orders. The external walls were originally covered in travertine, and marble statues once filled the niches on the 2nd and 3rd storeys. The upper level, punctuated with windows and slender Corinthian pilasters, had supports for 240 masts that held up a canvas awning over the arena, shielding the spectators from sun and rain. The 80 entrance arches, known as vomitoria, allowed the spectators to enter and be seated in a matter of minutes.

The Colosseum's interior was divided into three parts: the arena, cavea and podium. The arena had a wooden floor covered in sand to prevent the combatants from slipping and to soak up the blood. Trapdoors led down to the underground chambers and passageways beneath the arena floor – the hypogeum. Animals in cages and sets for the various battles were hoisted onto the arena by a complicated system of pulleys. The cavea, for spectator seating, was divided into three tiers: knights sat in the lowest tier, wealthy citizens in the middle and the plebs in the highest tier. The podium, a broad terrace in front of the tiers of seats, was reserved for emperors, senators and VIPs.

With the fall of the empire in the 6th century, the Colosseum was abandoned. In the Middle Ages, it became a fortress occupied by two of the city's warrior families: the Frangipani and the Annibaldi.

Damaged several times by earthquakes, it was later used as a quarry for travertine and marble for Palazzo Venezia, Palazzo Barberini and Palazzo Cancelleria among other buildings. Pollution and vibrations caused by traffic and the metro have also taken their toll.

The top tier and hypogeum have recently been opened to the public. Visits, which cost €8 on top of the normal Colosseum ticket and are by guided tour only, require advance booking.