L'Arène de Lutèce. The old Roman amphitheater was recovered and restored in the 19th century after a tram company started to build on the site. Some of the arena is original. Other parts, such as the stands, are restored. The base of the stage is in the foreground. You can see the rectangular openings for the animal cages in the midground.

A model of the arena. It could seat around 17,000 people. It was unusual in having a stage. This let the amphitheater be used for fights and for performances. (Photo source: Wikipedia)

Approaching the arena through a vomitorium. This was originally a corridor under the seats.

The arena is still used for games.

The city wall built (1190-1220) in the reign of King Phillipe-Auguste was pretty solid (some of it came from the ruins such as the arena's). Remains can be seen throughout the city. I came across this one in the Latin Quarter on rue Clovis. (Here's a web site with a map of the wall in 1210 and maps and photos of the modern remnants on the left and right banks.)

I walked around the block to see if it came out the other side. It didn't. But on rue du Cardinal Lemoine an open gate revealed the old wall in use as the rear wall of a private courtyard.