The Castle of Saint Angelo was not originally built for defensive purposes, but as the Mausoleum of Hadrian. As early as 403 C.E. the emperor Honorius may have incorporated the building in an outpost bastion of the Aurelian walls. In the 10th century it was transformed into a castle.
Submitted by Paul Young for ARCH 600: History of Ancient and Medieval Architecture.
Jordan, R. Furneaux. (1962). European architecture in color from the Greeks to the ninteenth century. London: Thames and Hudson. pl. 15, p. 65-66