Colosseum

The Flavian Amphitheatre, commonly known as the Colosseum, is the largest Roman amphitheater in the world and a must-see if you are passing through Rome.

The Coliseum (ancient name: Amphitheatrum Novum or Amphitheatrum Flavium, Italian: Colosseo) is the largest amphitheater built in ancient Rome, the largest enclosed building of Roman antiquity and still the largest amphitheater ever built in the world. Built between 72 and 80 AD, the Coliseum served as the venue for most of the most gruesome and brutal events hosted by members of the Imperial House for the entertainment and amusement of the free citizens of Rome and the Roman Empire, with free entry. Today, the ruin of the building is one of the landmarks of the city and at the same time a testimony to the ancient architecture of the ancient Romans.

Source; Wikipedia

2 day ticket for Colosseum and Roman Forum costs 12 EUR and can be purchased at the cash desk directly at the Colosseum.
As of Dec. 2017

Probably the most famous monument in the world and a symbol of the greatness of Rome, the Flavian Amphitheatre, known especially as the Colosseum for the colossal bronze statue of Nero that stands nearby, rises in the archaeological heart of Rome. the city, and for almost two thousand years it has told an uninterrupted story full of charm and magnificence.