The Castel Sant'Angelo, also known as the Mausoleum of Hadrian, is located in the district of Borgo, in Rome. This cylindrical building was built on the right bank of the river Tiber between 135 and 139 AD. The ashes of the Roman Emperor Hadrian, who died in 138 AD, were placed there when the building was completed. Until the year 217 AD, the ashes of other Roman Emperors were also placed here, a room deep inside the building's interior.

This imposing structure consists of a quadrangular basement in shape—each side is almost 90 meters long, and this part of the building is 15 meters high. The cylinder which sits at the top of this structure is 64 meters in diameter and 21 meters high. The mausoleum was originally adorned with a roof-top garden, a statue of Hadrian, and a bronze statue of a four-horse chariot called a quadriga. The whole was enclosed in a high wall with bronze gates, decorated with peacocks (these are thought to have been a funerary symbol). These are now gone—the top of the fortress is now adorned by a bronze statue of an angel, built by Flemish sculpture Pieter Verschaffelt in the eighteenth century. This statue represents an angel who, it is said, manifested atop the fortress in 590 AD and performed a miracle, ending a plague that had infested Rome. It is after this event that the structure became known as the Castel Sant'Angelo.

The Castel Sant'Angelo is connected to the rest of the city via the Ponte Sant'Angelo, also known as the Bridge of Hadrian. The bridge is decorated with exquisitely-detailed marble statues depicting angels in various poses—these were not part of the original bridge but were added during the Baroque period in the seventeenth century.

In the year 401 AD, the mausoleum was converted into a military fortress. Since then, much of the building's decoration and contents have been lost—even the ashes of the Emperors were scattered by Visigoth (a tribe that originated in East Germany) looters in 410.

In the thirteenth century, the structure was acquired by the Papacy and converted into a Papal castle. In 1277, the structure was occupied by Pope Nicholas III, under whose rule the mausoleum was connected to the Vatican by means of a fortified corridor known as the Passetto di Borgo. This 800-meter-long tunnel served as an escape route for endangered Popes on numerous occasions. In 1494, for example, the tunnel was used by Pope Alexander IV to escape from the French king Charles VIII. The fortress was also a refuge for Pope Clement VII when Rome was sacked in 1527. Clement VII was another Pope who made use of the Passetto di Borgo.

A chapel was added to the structure by Pope Leo X, and later richly-decorated rooms were added by Pope Paul III, intended to be a safe residence for future Popes if the city was besieged again. These rooms are adorned with frescoes and other decorations. Later, the Castel Sant'Angelo was used by the Papal state as a prison—among others, Giordano Bruno was imprisoned there for six years, from around 1593 to 1599, for heresy, blasphemy, and immoral conduct, which included his belief that the Earth revolved around the sun, rather than the opposite situation which was part of church dogma at the time.

In 1901, the Castel Sant'Angelo was decommissioned, and the castle became the museum known as the Museo Nazionale di Castel. The castle is divided into five floors. The first floor is simply an entrance point and is the start of an enormous winding ramp, approximately 400 feet long, which you will walk up to reach the upper floors. On the second floor are rooms that were used as prison cells and storage rooms. The third-floor houses two large courtyards, and on the fourth floor are the rooms used by various Popes over the years as places of refuge. The fifth floor is an open terrace that offers magnificent panoramic views of the city.

The Museo Nazionale di Castel is located at Lungotevere Castello, 50 in the Borgo district of Rome and is located easily from the city center. It is open from 9 am every day, excluding Mondays. Tickets cost between 2.5 and 5 Euros.