On the right bank of the Tiber, in the heart of Rome, stands the ancient sepulchre of Hadrian, nowadays known as Castel Sant'Angelo. In 590 Pope Gregory the Great, conducting a penitential procession to pray for the end of the plague, had a vision of the archangel Michael sheathing its sword over the castle, signifying the end of the plague; from this event the structure takes its modern name and the building is surmounted by a bronze statue of the archangel. Work on the edifice together with the facing Pons Aelius began in 123 under the emperor Hadrian and was finished in 139 AD by Antoninus Pius. The mausoleum contained the earthly remains of the emperors up to Caracalla. Later the building was incorporated into the defensive system planned by Aurelian and transformed into a fortress, while Theodoric used it as a gaol. Further strengthened, for Nicholas V, with the addition of bastions, towers and a defensive wall, the castle housed the archive and treasure of the Church, before becoming the pontifical stronghold and residence from the sixteenth century onwards, when Pope Alexander VI Borgia engaged the architect Antonio da Sangallo to modify the structure. Restructured and embellished several times by pontiffs in successive centuries, and restored at the end of the nineteenth century, Castel Sant'Angelo is a massive round building, surrounded by four bastions, surmounted by a square tower. The superb complex of Renaissance pontifical apartments and the imposing entry portal created in 1556 by Giovanni Sallustio Peruzzi are outstanding. The castle houses the important National Museum, where a rich collection of arms can be admired, the cells destined to prisoners, interesting documents about the history of the building, as well as the above-mentioned pontifical apartments, with their wealth of extraordinary works of art. Among these is a fine St. Jerome in the desert, masterpiece of Lorenzo Lotto, and the great painting by Luca Signorelli depicting the Madonna and Child and the saints in the splendid rooms of Clement VII; the Paolina Room is also admirably decorated by Perin del Vaga, part of the apartment of Paul III; a sixteenth-century marble high relief portraying the Virgin and Child, is a fine work by Raffaello da Montelupo in the Chapel of Leo X .Finally, the loggia of Julius II, probably the work of Bramante, adorned by magnificent columns, and the Room of the Hadrianeum, with its lovely frieze with mythological scenes alternating with descriptions of ancient Roman monuments.