Castel Sant’Angelo,also called Hadrianeum , or Sepulcrum Antoninorum, structure in Rome, Italy, that was originally the mausoleum of the Roman emperor Hadrian and , until Caracalla, became the burial place of the Antonine emperors until Caracalla. It was built in AD 135–139 and converted into a fortress in the 5th century. It is stands on the right bank of the Tiber River and guards the Ponte Sant’Angelo, one of the principal ancient Roman bridges. In plan, the fort is a circle surrounded by a square; each corner of the square is protected by an individually designed barbican, or outwork, while the central circle is a lofty cylinder containing halls, chapels, apartments, courtyard, and prison cells.
