cavetocanvas:

Giovanni Battista Piranesi, View of the Mausoleum of Hadrian (now called the Castel S. Angelo), c. 1756

From the Heilbrunn Timeline of Art History:

Piranesi’s earliest views of Rome, such as the Piazza del Popolo, had placed the principal buildings at the back of a vast, tilted space, filled with a diversity of human activity. By the mid-1750s, the monuments fill the space more commandingly, and are seen as if from below and close at hand, as in this view of the Castel Sant’Angelo. Piranesi produced this view at the same time he was working on theAntichità Romane, his four-volume archaeological treatise, and it may have been originally intended for that work. The clear didactic character of the image and the type of lettering are characteristic of the Antichità Romane, although the dramatic view of the castle as it appeared in the eighteenth century is entirely in keeping with the other Vedute. The buildings at the top of the structure served as a gracious and well-fortified refuge for the pope in times of trouble—the corridor that stretches to the right atop huge arches leads all the way to the Vatican Palace.