Originally commissioned by the Roman Emperor Hadrian as a mausoleum for himself and his family the building was later used as a fortress and castle, and is now a museum.

A new bridge (called Pons Aelius from the name of the emperor) which still exists as Ponte S. Angelo was built across the Tiber in order to reach the Campus Martius. Most of the structural parts of the Mausoleum, which was incorporated into Castel S. Angelo in the Middle Ages, have been preserved. The building consisted of an enormous quadrangular basement, 89 m. each side and 15 m. high. On top was a cylindrical drum flanked by radial walls. A bulk of earth planted with trees rose up over the drum. Along the edges were decorative marble statues and at the centre, raised even higher up, was a podium on top of which was a bronze quadriga crowned with the statue of Hadrian. The Funeral chamber, right at the center of the massive drum, is still visible:the cinerary urns of the emperors were placed in this room.

As early as A.D. 403 the emperor Honorius may have incorporated the building in an outpost bastion of the Aurelian walls. In 537, when it was already a fortress, it was attacked by Vitiges and the Goths. In the 10th century it was transformed into a castle. Its appearance today is that of a massive fortress on a square base and with circular towers at the four corners onto which a circular body has been grafted. This was built following the lines of the Imperial mausoleum under Benedict IX. Further work was ordered by Alexander VI and by Julius II who had the south loggia above the papal apartments added.

At the summit is the panoramic terrace, watched over by the Angel about to fly off, which seems to be why the building is called as it is, for the winged messenger is said to have saved Rome from a terrible plague at the time of Gregory the Great.