The amphitheatre was built outside the camp wall on the NE side in the first decades of the 3^rd cent. A.D. by the masons of the Legio Albana. This massive oval structure was in part dug out of the rocky  bank and in part constructed with various technique being used.

All that is left of the original building is the first floor supported by 30-odd arches, part of the triumphal entrances and the whole cavea, which measures 113 m. along the main axis.

Originally the amphitheatre reached a height of about 22 m. In the Middle Ages it was exploited as a source of building materials and was also used as a Christian  cemetery. As a reminder of this latter phase  there are two oratories, one of them obtained by converting the third archway  and the other carved entirely in the rock and the left side of the parapet of the cavea, on a level with the arena.

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