Identify.

Hadrian's Mausoleum, now Castel Saint'Angelo, located not far from Mausoleum of Augustus, concrete drum likely with earth piled high (like Etruscan Tumulus, using Italic forms, top resembles Hellenistic architecture [original]), originally topped with statue of Hadrian in a Quadriga, later a papal fortress and museum, not 100% how the inside was used and how the emperors were buried there, Emperors who did bury themselves here were tying themselves to previous emperors, repurposed marble thought to be decoration inside St. Peter's Basilica