The Mausoleum of Hadrian, more often known as Castel Sant'Angelo (pronounced [kaˈstɛl sanˈtandʒelo]; Italian for 'Castle of the Holy Angel'), is a towering rotunda (cylindrical building) in Parco Adriano, Rome, Italy.

Hadrian also built the Pons Aelius facing straight onto the mausoleum – it still provides a scenic approach from the center of Rome and the left bank of the Tiber, and is renowned for the Baroque additions of statues of angels holding aloft instruments of the Passion of Christ.

[3] Much of the tomb contents and decorations have been lost since the building's conversion to a military fortress in 401 and its subsequent inclusion in the Aurelian Walls by Flavius Honorius Augustus.

An unusual survivor, however, is the capstone of a funerary urn (probably that of Hadrian), which made its way to Saint Peter's Basilica, covered the tomb of Otto II and later was incorporated into a massive Renaissance baptistery.

[4] The use of spolia from the tomb in the post-Roman period was noted in the 16th century – Giorgio Vasari writes: ...in order to build churches for the use of the Christians, not only were the most honoured temples of the idols [pagan Roman gods] destroyed, but in order to ennoble and decorate Saint Peter's with more ornaments than it then possessed, they took away the stone columns from the tomb of Hadrian, now the castle of Sant'Angelo, as well as many other things which we now see in ruins.

[5]Legend holds that the Archangel Michael appeared atop the mausoleum, sheathing his sword as a sign of the end of the plague of 590, thus lending the castle its present name.

He recounts that during a prolonged season of the plague, Pope Gregory I heard that the populace, even Christians, had begun revering a pagan idol at the church of Santa Agata in Suburra.

Returning to St Peter's by the Aelian Bridge, the pope had another vision of an angel atop the castle, wiping the blood from his sword on his mantle, and then sheathing it.

[11] There is an ongoing project to connect Castel sant'Angelo to the St. Peter's Basilica via a fully pedestrian path, by creating an underground tunnel under the Piazza Pia Place.