Built as the mausoleum of the emperor Publius Aelius Trajan Hadrian (76-138 AD) and a dynastic sepulcher for the Antonini family, first with the emperor Aurelian and then with Honorius, the imposing mass was included in the walls of Rome and transformed in a sort of fortress for the defense of the city.For these prerogatives, since then, it acquired the name of castellum, to which was added, in the early Middle Ages, that of sancti Angeli from the legend of the vision of the Archangel Michael who the sword to bear witness to the end of the plague.