Arenas and Ruins
Long ago, The Roman Empire was home to something known as the gladiator games. These such "games," were held in places called arenas. What is an arena, you may ask? An arena is a large, circular or oval open-air venue with raised seating, where the gladiator games and martyrdom would take place. Some arenas could hold anywhere form 10,000 people, to 20,000 people. They were also used for venationes (animal slayings). The name "amphitheatre," means "theatre all around," which is why they are oval shaped. These are estimated to have originated somewhere around 70 A.D. The largest of the arenas was the "Colliseum." This could hold an estimated 50,000-80,000 people, but on average, was occupied at 65,000 capacity.
This image of thr Arles Arena is actually a full-blown depiction of an Arena that Perpetua and Felicitas could have been martyred in. During that time period, an arena like this was used for all sorts of things, such as: Martyrdom, Gladiator games, and venationes (animal slayings.)
This amphitheatre at Carthage was actually in existence during the time that Perpetua and Felicitas were martryed. It could possibly be the one where their martrydom took place.