Built, as was the Elian Bridge in front, by the Emperor Hadrian (117-138) as a Mausoleum for himself and his successors, it was completed by Antoninus Pius in 139. In 271, the Emperor Aurelian incorporated the pile into the defence system he designed: it lost its function as a tomb to become a fortress.
In 1277 it was occupied by Nicholas II who connected it to the Vatican by the famous corridor, a safety passage which runs along the top of the encircling wall of the Vatican. Hencefort, it remained under the control of the Popes who used it as a fortress, to impress, but also as a prison and a place for torture.