This imposing building was erected in 135 AD as Emperor Hadrian's family mausoleum and was converted into a chapel by Pope Gregory the Great in the 6C. It acquired a brick layer and keeps under Pope Nicholas V (15C) and octagonal bastions under Alexander VI. A fine helical ramp leads to the castle which is crowned by elegant papal apartments. From the terrace at the top, there is a splendid panorama of the city.