The Castel Sant'Angelo, also known as the Mausoleum of Hadrian, is a prominent monument in Rome. It is located on the right bank of the Tiber River, opposite the Pons Aelius and a short distance from the Vatican, between the Borgo and Prati districts. This castle is connected to Vatican City through a fortified passageway known as the Passetto. Throughout its history, the castle has undergone significant transformations during the medieval and Renaissance periods.

Currently, it is owned by the Italian State, and since December 2014, its management has been in charge of the Ministry of Cultural Heritage and Activities through the Lazio museum hub. As of December 2019, oversight became the responsibility of the Directorate of State Museums of Rome.

The origin of the castle dates back to 359 when Emperor Honorius incorporated it as part of Rome's walls, turning it into a sort of fortress for the defense of the city, at which point it acquired the name "castellum." In the year 590, it was named "castellum sancti Angeli" in memory of a vision of the archangel Michael had by Pope Gregory the Great during a penitential procession to dispel the plague ravaging Rome. This vision was interpreted as a portent of the imminent end of the epidemic, which quickly came true.

In 974, the castle was seized by Crescenzio, a member of the Alberico family, who further fortified it. For this reason, it was renamed "Castrum Crescentii," a name that lasted until the second half of the 15th century when its current name was finally adopted. Before the 11th century, it was known as "Adrianeum," and it was also mentioned as "templum Adriani" and "templum et castellum Adriani," referring to its origin planned by Emperor Hadrian, who intended to use it as an imperial tomb for himself and his successors. The memory of these names is reflected in the current name of "Mole Adriana."

From the 11th century, in papal bulls, mixed terms such as "Castrum nostrum Crescentii" and "Castrum Sancti Angeli" were used. In the songs of gesta, it was also called "Tower" or "Palais Croissant," which is a translation of "Crescentii" and, in its literal sense, means "palace in the shape of a crescent moon." Before the year 1000, chroniclers mentioned it as "domus Theodorici" and "carceres Theodorici" because Theodoric, king of Italy from 493 to 526, dedicated it to prison, a function it retained under the rule of the popes and during Italian governance until 1901.