Castel Sant'Angelo in Rome was originally created as a stately mausoleum by the will of the Roman Emperor Hadrian between 135 and 139 AD.
Over the centuries it has seen its share of changes from the original mausoleum to a fortress, then a castle, to its present day function as a museum.

The ashes of Hadrian were the first to take their eternal resting place here when his tomb was completed a year after his death. His remains were moved to Italy from Italica, Baetica, now Spain, and were cremated by his second son and successor Antoninus Pius. Later, those of his wife Vibia Sabina and first son Lucius Aelius, were also placed there.
Other emperors also reserved their places in the mausoleum with the last recorded being Caracalla in 217. The urns containing these ancient ashes are now all found deep within the structure in a room called the Treasury.
A very interesting architectural aspect of Castel Sant'Angelo is the Ponte or bridge, best noted for its statuary of angles holding elements of the Passion of Christ, added when it was restructured during the Baroque period.
In 401 the mausoleum saw its first radical changes when it was converted into a military fortress. Unfortunately, during the sack of Rome in 410, the urns were ransacked by looters and as recounted by the historical writer Procopius, the original decorative bronze and stone statuary was also destroyed when it was thrown down on the attackers during the siege by the Goths in 537.
Centuries later, the fortress was converted into a castle for the popes when Pope Nicholas III had it connected to Saint Peter's Basilica by a covered corridor named Passetto di Borgo.
During the plunder of Rome in 1527, Pope Clement VII took refuge there. Later still, Pope Paul III had a luxury apartment built there to insure that during any future sieges that a Pope would have a more dignified place to hide.
The grandiose top of the mausoleum is mounted with a statue of the archangel Michael sheathing his sword symbolizing the end of the plague of 590.
Castel Sant'Angelo has been a museum, Museo Nazionale di Castel Sant'Angelo, since 1901 and can be visited daily.