What Roman mausoleum in Rome was later converted into the Castel Sant'Angelo?

The Roman mausoleum in Rome later converted into Castel Sant’Angelo is the Mausoleum of Hadrian.

Emperor Hadrian commissioned the enormous cylindrical tomb on the right bank of the Tiber, and construction began around AD 135. The structure was completed in 139, shortly after Hadrian’s death, and it became the burial place of Hadrian and several later Roman emperors. Its original design included a square base, a massive cylindrical drum, a garden or upper platform, and a crowning statue of the emperor in a chariot.

During late antiquity and the Middle Ages, the mausoleum was incorporated into Rome’s defensive system. Its transformation accelerated after the Aelian Bridge and the surrounding fortress works connected it to the city. In medieval tradition, the building acquired the name Castel Sant’Angelo after a vision of the archangel Michael was said to have appeared above it during a plague.

The building is therefore both a funerary monument and a fortress. The Mausoleum of Augustus is a different imperial tomb, located elsewhere in Rome and not the structure that became Castel Sant’Angelo.