View of the Ponte and Castel S. Angelo

The Castel Sant' Angelo was originally built as the Mausoleum of Hadrian (117-138), begun by the emperor in the 120s and completed one year after his death. The tomb consisted of a square base with marble statues of men and horses at each corner. Set within and rising above the square was a circular drum of solid concrete pierced by a spiraling ramp leading to the burial chamber itself. On top of this drum was a central tower that supported, it is thought, a monumental statue of either Hadrian or a four-horse chariot. The mausoleum was entirely encased in white marble and rose to a height of over fifty meters. Only the bare outlines of this structure exist today. Over the centuries it was heavily fortified, then used by various factions as a prison and citadel, and gradually stripped of its ornament. Many parts were demolished and reconstructed. By the twelfth century it had been dedicated to the archangel Michael, whose statue now stands on top, and recognized as the property of the papacy. It was to continue as a retreat and refuge for popes until the nineteenth century. It became a museum in the first half of the twentieth.

The bridge was built by Hadrian to link his tomb to the city across the Tiber. The arches in Piranesi's view are the original ones, but the parapet now bears ten angels, designed by Bernini and executed by his pupils in the 1600s. Statues of Saints Peter and Paul also stand at one end. The bridge and the Castel frame the imposing dome of St. Peter's beyond, rendering a memorable image of the center of power of the Catholic Church.