September 29th is the feast day of the Archangel Michael, depicted as a warrior with armor and sword, defeating the devil. He is venerated as the leader of the heavenly armies, protector of humanity against evil and Satan, champion of justice, and defender of the faith.
The Archangel has a special relationship with Castel Sant'Angelo because, according to legend, he appeared to Pope Saint Gregory the Great in 590 AD, while he was leading a penitential procession during the terrible plague of those years, which had caused millions of deaths.
When the pope was on the Pons Aelius (the Aelius Bridge), the Archangel appeared to him on the top of the Castle, wiping the blood from his sword and sheathing it: a divine sign that the plague was over. From that moment, the Mausoleum took the name Castel Sant'Angelo.
The Mausoleum of Hadrian originally had a Temple at the top, crowned by an enormous Quadriga of the Sun driven by the emperor, depicted as Sol Invictus (the Sun god who defeated darkness), who guided the destinies of the world.
In the 13th century, the chapel of Sancti Angeli usque ad coelo (Saint Angel near the Sky) was created inside the Temple to commemorate the apparition, and in the 15th century, a statue of the Archangel was placed on top of the Castle.
Interestingly, according to tradition, Archangel Michael was the solar Archangel par excellence: «beautiful, shining with light, bathed in victory, the protector from the snares of the dark forces, stronghold of Light and bulwark against Darkness».
There was therefore a continuity between pagan and Christian religious symbols: the Emperor and the Archangel were both linked to the Sun, to Light, and to the triumph over Death and Darkness, and both were depicted as invincible warriors.
This and much more about the Castle's fascinating, millennia-long history can be found in our book, «Castel Sant'Angelo. Mausoleum of Hadrian's. Architecture & Light» published by Rirella Editrice, also in an English edition. It is available for purchase on the Rirella Editrice website (link) and in Rome at the Castel Sant'Angelo National Museum Bookshop.