The Castle Sant' Angelo was originally a round tomb built on the Tiber River to hold the remains of the Emperor Hadrian. The Hadrianeum or Hadrian's Mausoleum was begun by Hadrian in 130 AD and finished in 139, a year after his death. During the middle ages, Hadrian's tomb was converted into a castle to defend Rome. It was linked to the Vatican by an underground passage. Many Popes lived in this castle.