Castel Sant' Angelo (kästel' säntän'jālō) [key], Hadrian's Mausoleum,or Hadrian's Mole,massive round construction on the right bank of the Tiber in Rome. Originally built (A.D. 135–39) by Emperor Hadrian as a mausoleum for himself and his successors, it was later decorated and fortified as a place of refuge for the popes and was connected to the Vatican by a secret passage. It was used as a fortress and prison until 1870 and is now a museum.
