Roman Emperor Hadrian, who ruled from 76 to 138 AD, commissioned this enormous circular structure along the Tiber River to be his mausoleum. He died a year before it was completed but his ashes were interred here in 139 AD. Over the centuries, Castel Sant’Angelo has served as a military fortress, a citadel for popes and a prison. Since 1901, it has been the Museo Nazionale di Castel Sant’Angelo. Notice the bronze statue on top. According to folklore, Archangel Michael appeared over the castle in 590 AD during a procession led by Pope Saint Gregory. This apparition signified the end of the bubonic plague that was devastating Rome.