The Castrensian Amphitheatre was a modest amphitheatre of the 2nd century AD in which games were probably staged for soldiers. (The camp of the imperial horseguard was close to the site.)
In the 3rd century it was incorporated into the Aurelian Wall with the arches lying beyond the wall being filled in. On the picture below and on the picture of the model, you should just be able to make out the filled in arches.
Model of Rome in the days of Constantine the Great
Museo della Civilta, Rome