Hadrian´s Mausoleum and Pons Aelius, view from Ponte Vittorio Emanuele II, the next western Tiber brige in Rome

The Mausoleum of Hadrian, usually known as Castel Sant'Angelo (English: Castle of the Holy Angel), is a towering cylindrical building in Parco Adriano, Rome, Italy. It was initially commissioned by the Roman Emperor Hadrian as a mausoleum for himself and his family.
The tomb of the Roman emperor Hadrian was erected on the right bank of the Tiber, between 134 and 139 AD. Originally the mausoleum was a decorated cylinder, with a garden top and golden quadriga. Hadrian's ashes were placed here a year after his death in Baiae in 138, together with those of his wife Sabina, and his first adopted son, Lucius Aelius, who also died in 138. Following this, the remains of succeeding emperors were also placed here, the last recorded deposition being Caracalla in 217. The urns containing these ashes were probably placed in what is now known as the Treasury room deep within the building. 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 Augustus Honorius. The urns and ashes were scattered by Visigoth looters during Alaric's sacking of Rome in 410 AD., and the original decorative bronze and stone statuary were thrown down upon the attacking Goths when they besieged Rome in 537.
The building was later used by the popes as a fortress and castle. Pope Nicholas III connected the castle to St Peter's Basilica by a covered fortified corridor called the Passetto di Borgo. The fortress was the refuge of Pope Clement VII from the siege of Charles V's Landsknechte during the Sack of Rome (1527 AD.). Now it is a museum, the Museo Nazionale di Castel Sant'Angelo, and a highlight for tourists.
Hadrian also built the Pons Aelius facing straight onto the mausoleum.

more on https://en.wikipedia.org/wiki/Castel_Sant'Angelo