Mausoleum of Hadrian

By N.S. Gill, About.com Guide

Now, the Castel d'angelo 'Castle of St. Angelo', this mausoleum was built by Hadrian in 135 to be his tomb. There is now a St. Michael statue on the top instead of the original quadriga and colossal statue of Hadrian. The basement was built as a square with each side 340 feet. Over it was a colossal circular tower, several stories high. The lowest story had a diameter of 230 feet. The exterior was columned and held statues. The mausoleum was later used as a fortress.

Source: A handbook of Architectural Styles, by Albert Rosengarten; translated by W. Collett-Sandars (1876).