The Mausoleum of Hadrian, also called Hadrianeum, is described by ancient sources such as Cassius Dio, who explains that it was built by the emperor Hadrian because there was no more space in the Mausoleum of Augustus.
The Mausoleum of Augustus was in the Campus Martius, but even there the space was not enough for a grandiose building like the one the emperor was thinking of.

Therefore he chose an area on the other bank of the Tiber, where the Horti Domitiae were located, which belonged to the Imperial House since a long time.

To reach that area he had the Aelius bridge specially built, which was the monumental and scenic access to the Mausoleum, and was inaugurated as early as 134 AD by Hadrian himself.

The Mausoleum, however, was completed in 139 AD, a year after the death of Hadrian, and the following emperors at east up to Caracalla were buried there, together with other members of the imperial family.

After centuries of abandonment and numerous lootings by the Visigoths, the Mausoleum, was reduced to a tower and stripped of all its marbles, and became a strategic element in the defense of the city.

Its name changed in 590 AD. on the occasion of a terrible plague, when Pope Saint Gregory the Great carried the image of the Madonna Salus Populi Romani in a penitential procession from the basilica of Santa Maria Maggiore to that of Saint Peter.

When he crossed the Tiber on the Aelius Bridge, angelic voices were heard and the Archangel Michael appeared to him on the top of the Mausoleum, in the act of sheathing his sword: a divine sign that the plague was about to end. From that moment the bridge became Pons Sancti Angeli, i.e. Ponte Sant'Angelo, and the Mausoleum, now transformed into a Castle, took the same name.

You can read about this legend this and much more in Marina de Franceschini's new book "Castel Sant'angelo. Mausoleo diAdriano. Architettura e Luce".