The massive fortress of Castel Sant'Angelo takes as name from the vision that Pope Gregory the Great had of the Archangel Michael on this site. It began life in AD 139 as Emperor Hadrians mausoleum. Since then it has had many roles: as part of Emperor Aurelians city wall, as a medieval citadel and prison, and as the residenceof the popes in time of political unrest. From the dank cells in the lower level to the fine apartments of the Renaissance popes above, a 58-room museum covers all aspects of the castle's of the castle's history.
The Vatican Corridor leads from the Vatican Palace to Castel Sant'angelo. It was built in 1277 to provide an escape route when the pope was in danger. The pentagonal ramparts built around the the castle during the 17th century improved its defences in times of defences in time of siege.