The remains of the Castrensian amphitheatre are adjacent to the Basilica and the Monastery. The builiding is probably attributable to the the Emperor Eliogabalo's age (218-222). It was located on the most elevated place of the area and it was included by Aureliano (270-275) in the new defensive walls planned around the Eternal City.
It's name comes from Castrum that, in late ancient age, took on also the meaning of imperial abode. It was used for court performances and military drills. It has a circumference of about 250 m and it built in three orders of arches. The first order is marked by barrel-vaults, between pillars, framed by semi-columns; the second, of which we can see the remains near the Cappella del Buon Aiuto, followed the same scheme, whilst the third had windows divided by pillars with brackets to hold up the beams of the awning.
The remains of the Amphitheatre sustained considerable damage from the half of the XVI cent., when Paolo IV ordered the lowering of the part incorporated in the Aurelian Walls till the first order, due to defensive reasons.
Later on, the remains of the Amphitheatre followed the vicissitudes of the Monastery that used its materials in order to raise new buildings around the half of the XVIII cent.
Actually, with Benedetto XIV, the what remained of the terrace was demolished.
Recently, aimed excavations have identified, under the arena, the ensemble of services made of a central gallery on which areas with cross vaults overlook. Other two galleries branch off the main gallery with a semicircular course.