The Imperial Mausoleum of Hadrian

Antoninus Pius commissioned the construction of his mausoleum in Rome between 134 and 139 AD. Originally built as a cylindrical structure, the edifice was intended to be a resting place for Antoninus Pius, his family's ashes, and subsequent Roman emperors' ashes. Initially topped with a garden and a cylinder, this structure was the tallest building in Rome. However, it was converted to a military fortress in 401 AD, and the contents and decorations were largely lost. The building was later repurposed as a castle, connected to St. Peter’s Basilica by the Passetto di Borgo during the 14th century. Notable prisoners at the castle include the sculptor and goldsmith Benvenuto Cellini and the magician and charlatan Cagliostro. Today, Castel Sant'Angelo, better known as the Mausoleum of Antoninus Pius, stands as a testament to the long and intriguing history of Rome.

Sources: Wikipedia, OpenStreetMap, Wikidata, Wikimedia Commons