9. MAUSOLEUM of Hadrian (as converted into the Castel Sant'Angelo) and Aelian Bridge (135 CE) Burial site, constructed for Hadrian, his family, and future emperors, near Tiber River. opposite the field of Mars. Cylindrical (21 meters high and 64 meters in diameter) Square base. Became a papal fortress in the 6th century and reconsecrated as Castel Sant'Angelo. Became increasingly fortified (battlements added in the 11th century). Lost its marble facing and most of its friezes. It was the second largest building in ancient Rome (after the Colosseum ).