Castel Sant'Angelo, one of the most important monuments in Rome, located on the right bank of the Tiber River, was originally built as a mausoleum for Emperor Hadrian around 123 AD. Later in the course of history, it became a fortress, a papal residence, a prison and finally, today, a museum.

Built opposite the Campus Martius as a tomb of for Emperor Publius Aelius Trajanus Hadrianus (76-138 AD) and as the Antonine family's tomb, around 403 AD the monument was included in the Aurelian Walls by Western Roman Emperor Honorius and became a sort of fortress for the defense of the city. Because of this function, it is called "castellum," or castle. Several Roman families contended with each other for its possession: the Crescenzi, the Pierleoni and the Orsini.

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Fonte: Italy24