As stated in the introduction of Lyon, the city was founded by the Romans in the year 43 BC. They created a whole city out of nothing and called it Lugdunum. The city was built on the Fourviere hill located in Old Lyon (“Vieux Lyon“), the hill that today not only sees much of the Roman architecture, but also the Basilica and world’s oldest but still active funicular lines. And as Romans in those days wanted entertainment, they also created a theatre. And what a theatre it is!
The exact date of the construction of the theatres (the plural is correct, since you’ll see there are two theatres) is not know, but scientists estimate it was around 15 BC. In fact, the Romans built two theatres, the initial one housed 5700 spectators.

In the 2nd century AC, more space was added, now capable of housing 10,000 spectators. In other words, it’s massive!

Every year, June and July, the theatre is host to the festival called “Les Nuits de Fourviere“. Some 130,000 spectators come to the big Roman theatre to watch the spectacle. Click here for more information about the festival and see the program.

Next to the main Roman theatre, there is another, smaller, theatre called Odeon.

The exact date of construction is not known, but they guess it to be mid-second century AC. The Odeon theatre was once covered by a roof. It has a seating capacity of 3,000 seats.

From both theatres you have a great view over Lyon city and the rest of the hill.
Summary
It’s quite a spectacle to see these ruins. It’s more than ruins, they are still in very good shape, even after 2,000 years of usage. But what makes this a great visit, is that it’s FREE; no admission charge. (there is a Roman museum close by, but that is not free).
TIP: Take the funicular (Saint Just line -Minimes stop) which stops 20 meters from the ruins. Or better yet, take the funicular to visit the Basilica and then walk down the 200 meters to see the ruins.