The Mausoleum of Hadrian, usually known as Castel Sant'Angelo is a towering cylindrical building in Parco Adriano, Rome, Italy. It was initially commissioned by the Roman Emperor Hadrian as a mausoleum for himself and his family. The building was later used by the popes as a fortress and castle, and is now a museum. The Castel was once the tallest building in Rome.
Much of the tomb contents and decorations have been lost since the building's conversion to a military fortress in 401 and its subsequent inclusion in the Aurelian Walls by Flavius Augustus Honorius. The urns and ashes were scattered by Visigoth looters during Alaric's sacking of Rome in 410, and the original decorative bronze and stone statuary were thrown down upon the attacking Goths when they besieged Rome in 537, as recounted by Procopius. An unusual survivor, however, is the capstone of a funerary urn (probably that of Hadrian), which made its way to Saint Peter's Basilica, covered the tomb of Otto II and later and was incorporated into a massive Renaissance baptistery. The use of spolia from the tomb in the post-Roman period was noted in the 16th century.
Legend holds that the Archangel Michael appeared atop the mausoleum, sheathing his sword as a sign of the end of the plague of 590, thus lending the castle its present name. A less charitable yet more apt elaboration of the legend, given the militant disposition of this archangel, was heard by the 15th-century traveler who saw an angel statue on the castle roof. He recounts that during a prolonged season of the plague, Pope Gregory I heard that the populace, even Christians, had begun revering a pagan idol at the church of Santa Agata in Suburra. A vision urged the Pope to lead a procession to the church. Upon arriving, the idol miraculously fell apart with a clap of thunder. Returning to St Peter's by the Aelian Bridge, the Pope had another vision of an angel atop the castle, wiping the blood from his sword on his mantle, and then sheathing it. While the Pope interpreted this as a sign that God was appeased, this did not prevent Gregory from destroying more sites of pagan worship in Rome.