The construction of this castle situated on the right bank of the Tibre, was ordered by Emperor Hadrian between 130 and 139, to become its mausoleum. In the 3th century, it was used as a fortress and then became a prison and finally a museum. Its name comes from a roman legend from the 6th century, during the plague in 590. The Pope Gregory I would have seen St Michael the archangel at the top of the castle, putting its sword into its scabbard to indicate the end of the epidemy. That is why St Michael staue stands at the top of the castle.