A huge circular edifice in Rome, 230 feet in diameter, built by Emperor Hadrian for his tomb in 136, and later converted into a citadel. It is connected with the Vatican quarter by the Bridge of San Angelo, also the work of Hadrian, and formerly contained the Archive of San Angelo founded by Pope Sixtus IV, Pope Leo X, and Pope Clement VIII to preserve the most important documents and titles of possession of the Roman Curia. Transferred to the Vatican in 1798, the collection is still kept separate under the name of Archive of the Castle.