Castel Sant' Angelo was built as a mausoleum for the emperor Hadrian (r. 117-138) and his family. Begun in the late 120s, the mausoleum was not quite ready when Hadrian died in 138, and was completed in the following year by his successor Antoninus Pius (r. 138-161).
The mausoleum took the form of a cylinder (66 m in diameter and 21 m high) on a square base (89 m wide and 12 m high). The cylinder was surmounted by a huge mound of earth planted with cypress trees. On its summit stood a bronze four-horse chariot (quadriga) with Hadrian at the reins.
The ashes of Hadrian, and his wife Sabina, were the first to be deposited in the mausoleum, followed by those of most of the Antonine and Severan emperors and their wives. The ashes of the emperor Caracalla (r. 211-217) were the last to be deposited.
With the building of the Aurelian walls in the late 3rd century (271-75), the mausoleum was transformed into a fortress. It acquired its present name following a vision by Pope Gregory the Great (r. 590-604). According to legend, the pope was leading a procession of people to pray for deliverance from an attack of the plague when he saw a vision of the archangel St Michael hovering over Hadrian’s tomb. St Michael was in the act of re-sheathing his sword, which the pope took to be a sign that his prayers had been answered.
Hadrian's mausoleum was duly renamed the Castel Sant’ Angelo. For several centuries the castle was crowned by a marble statue of the St Michael, the work of Raffaello da Montelupo (c.1504 - c.1567). In 1747 it was replaced by the bronze statue that we see today, the work of the Femish sculptor Peter Anton Verschaffelt (1710-93).
By the 12th century the Castel Sant’ Angelo was established as papal property. Nicholas III (r. 1277-80), the first pope to make the Vatican his home, created a long passageway between the castle and the Vatican. Known as the passetto, it was built above the defensive wall erected by Pope Leo IV (r. 847-855). In 1527 Pope Clement VII (r. 1523-34) used the passetto to escape from the Vatican during the infamous Sack of Rome (May 6th).