Castel Sant'Angelo, located on the right bank of the Tiber River in Rome, Italy, is a towering cylindrical building initially commissioned by Emperor Hadrian as a mausoleum for himself and his family. Its construction began in 123 AD and completed in 139 AD. Over centuries, it served various roles; from a fortress, a residence for popes (connected to Vatican State by the fortified corridor 'Passetto di Borgo'), a prison, to a museum. Its name 'Sant'Angelo' originates from the legend of Archangel Michael appearing atop the mausoleum, sheathing his sword as a sign of the end of the plague of 590. Visitors can explore its rich history, from Roman imperial tombs to lavishly decorated papal apartments, along with enjoying a panoramic view of Rome from the terrace. The castle's intricate history, coupled with its architectural transitions, reflects Rome's transformation over the millennia.