The 72-foot-high ruin of the facade of the Teatro Romano guards the remains of the 1st-century BC amphitheater, which once held 20,000 spectators. Only a bit of the outside wall and seven of the amphitheater's original 60 arches remain. The latter, once incorporated into medieval buildings, are being brought to light by ongoing archaeological excavations. The actual site cannot be visited, but you can get good view of it from the adjacent Via Baillage.
Via Anfiteatro 4, Aosta, 11100, Italy