The vision which Pope St. Gregory the Great had of St. Michael the Archangel
This image of Our Lady in Santa Maria Maggiore is venerated under the title Salus Populi Romani (Salvation of the Roman people) because Pope St. Gregory the Great had it carried in procession to pray to Our Lady for an end to the plague. When the procession reached Castel Sant’Angelo, the Pope had a vision of St. Michael the Archangel putting his sword back into the scabbard, indicating that the plague had ended.
The wall running from the Vatican to Castel Sant’Angelo as seen from the Vatican.
The wall running from Castel Sant’Angelo to the Vatican as seen coming from Castel Sant’Angelo