The exterior is of travertine, formed by 41 arches framed by pilasters, whereas the radial walls and the ambulatories were built in square work, consisting of blocks of tufa, cement work and reticulated work.
Today only the two lower stories in Doric and Ionic style are preserved. The stage was flanked by two rooms with apsis followed by a large exedra with two little temples.
It was originally 32 mt high and could hold about 15000 spectators.In the Middle Ages it was transformed into a fortress.
In the 1500's the noble family of Savelli transformed it into a palace (which was built by Baldassarre Peruzzi).
The building passed then to the Orsini family in the 1700's.