The Mausoleum of Hadrian, today’s Castel Sant'Angelo, was built in a strategic point of great passage and visibility, along the road that would later become the main itinerary of pilgrims towards St. Peter's; it crossed the Tiber with the Aelius bridge which for this reason was called Poncti Sancti Petri (St. Peter’s Bridge).

In 270 AD, when the construction of the Aurelian walls began, it seems that the Mausoleum was left outside the defensive walls, because it was located on the other bank of the Tiber.

Things changed after the battle of Adrianopolis in 378 AD, when the emperor Valens was defeated and killed by Alaric's Visigoths and the pressure of the barbarians on Italy became increasingly threatening (soon we will write about this crucial historical event).
While Milan was besieged in 401 AD, the emperors Honorius and Arcadius hastened to raise and strengthen the Aurelian walls, adding new towers. The Mausoleum of Hadrian was included in the new city wall, and became the most important defensive stronghold on the right bank of the Tiber.

In 410 AD there was the first major sack of Rome, by the Visigoths, followed by that of the Vandals in 455 AD. In both cases the barbarians failed to conquer the Mausoleum, but camped around it, creating a semi-permanent settlement which would later become the Borgo, and from there they devastated the entire city.
Since then the Castle remained an impregnable fortress, made increasingly larger and more powerful by the interventions of medieval owners such as the Crescenzi who owned it in the year 1000, and then by the various popes.

The Mausoleum was also called Hadrianeum, and at the beginning of the Aelius bridge the Gate called Sancti Petri in Hadrianio was built (Gate of Saint Peter’s near the Hadrianeum), which became one of the main gates of the city, to monitor access.
We have some engravings that depict it, because at the end of the fifteenth century Pope Alexander Borgia decided to further fortify the access to the Bridge, and without regrets destroyed part of the Aurelian walls including the ancient gate, of which no trace remains.

In that period the definitive transformation of the Mausoleum into a Fortress took place, due to its strategic position and its tower shape, which today is hidden by the Renaissance modifications. The same thing happened, for example, with the Mausoleum of Cecilia Metella on the Ancient Appian way, which in the 13th century was included in the Castrum Caetani, a fortress which controlled access to the city and therefore trade and commerce.