The imposing Castel Sant ' Angelo is a towering cylindrical building on the Tiber River in Rome, Italy.  It was originally commissioned by the Roman Emperor Hadrian as a mausoleum for himself and his family.  It has also been used  later by the popes as a fortress , a prison, a castle, and today as a military museum.    It was once the tallest building in Rome and the entire building  was covered with marble and many statues.   As was the practice in Rome, later rulers used the earlier buildings as their "quarries" and took the  marble and used it on their new buildings.

The mausoleum's construction originally began in 123 AD.  Hadrian died before the completion and his ashes were placed here in 138 AD, together with those of his wife and his first son.  The remains of succeeding emperors were also placed here, the last being Caracalla in 217.

The castle's strategic position , along with its extra towers and walls, made it an important outpost, and during the Middle Ages, it was transformed into a fortress.   By the XIV century, Castel Stant'Angelo had tied it's fate to that of the popes.  Pope Boniface IX even turned it into his residence and built a tunnel directly to St. Peter's.  Many later Popes used this tunnel as their "escape route".

At the top of the structure, stands a large statue depicting the archangel Michael, who, as the legend goes, miraculously ended the severe plague that has infested Rome.  Afterwards, the building was renamed Castel Sant ' Angelo, in his honor.

Most of the original contents and decorations of the castle have been lost during the various conversions.  Many of the statues were thrown into the Tiber.  Opera buffs know that Castel Sant'Angelo is the setting for the third act of Puccini's "Tosca".  The heroine , Floria, leaps to her death from the castle's ramparts.