Owing to its long history, the Castel Sant'Angelo is an extremely complex structure with strong contrasts in its architecture, sculpture and painting. From the outside it looks like a massive fortress, surrounded by a square wall with battlements and bastions at the corners. The cylindrical structure that rise above this, has in its core the mausoleum built by the 14th roman emperor Hadrian (AD 117-138) for himself and his future generations.
Originally, a cylindrical structure 21m in height and with a diameter of 65m it rose from a high square base of tuff and travertine blocks with side 86m long. This structure probably held statues and enclosed the burial chamber. It culminated in a round temple crowned by a bronze “quadriga” (four horse chariot) with the statue of the emperor. Inside, a spiral ramp that can still be used today, led to the burial chamber.
Just under two centuries later, the mausoleum was incorporated in the Aurelian city wall and it was strengthened to become a fortress. In the 6th century it fell into the hands of the Goths, and in the Middle Ages served as a citadel and a prison; in dangerous times it was a refuge for the popes who could reach the castle from St. Peter's down a roofed passage called the “passetto”.
Between the 10th and 14th centuries Castel Sant'Angelo was the only fortress in Rome and practically dominated the city, with the most powerful princes fighting for control of it. The popes demanded the return of the castle as one of the conditions for their return from Avignon and it finally into the hands of the Holy See in 1379.
After the last uprising against the papacy, conversion wor was started under Nicholas V (1447 – 1455) and his successors, who modernized the fortress to make it, as the papal residence, both safer and more comfortable. The castle was enclosed in a wall of fortifications with four bastions at the corners; a moat was dug and the “passetto” was fortified. A brick superstructure was added to the cylindrical building in several phases to house the magnificent papal apartments. Today these are used as a museum to display sculptures, paintings and weapons.