Best tours and activities for Castel Sant'Angelo

Located in the heart of Rome, between the banks of the Tiber River and the Vatican City, the Castel Sant'Angelo has a long and complex history, like many other monuments of the city. 
 
It was originally built, between 135 and 139AD, as the mausoleum of Hadrian. In 401AD, however, Castel Sant'Angelo was incorporated in the Aurelian Walls and then, during medieval times, the Popes started touse it a fortress to protect themselves in time of danger, as testified by the underground corridor that connects the building to the Vatican Palace.
 
Today, Castel Sant'Angelo houses in its interior a museum that retraces its history: from ruins of the Hadrian's tomb to remains of the fortified castle, the original prison cells and the papal apartments.