The Castel Sant'Angelo, or Mausoleum of Hadrian, is a mausoleum in a cylindrical shape, commissioned by the Roman emperor Hadrian for himself and his relatives. It was built between 135 and 139, but around 400 the mausoleum was converted into a military fortress. Later it also served as a prison. Nowadays it features a museum, the Museo Nazionale di Castel Sant'Angelo.