"This overpowering castle on the Tiber is Rome’s chief citadel and dungeon and has seen more blood, treachery, and turmoil than any other left in Rome. Even those on a rushed visit to Rome might want to spend some time here. It was built in the 2nd century as a tomb for Emperor Hadrian; it continued as an imperial mausoleum until the time of Caracalla. If it looks like a fortress, it should—that was its function in the Middle Ages. It was built over the Roman walls and linked to the Vatican by an underground passage that was much used by the fleeing papacy, who escaped from unwanted visitors such as Charles V during his 1527 sack of the city. In the 14th century, it became a papal residence, enjoying various connections with Boniface IX, Nicholas V, and Julius II, patron of Michelangelo and Raphael. However, its legend rests largely on its link with Pope Alexander VI, whose mistress bore him two children (those darlings of debauchery, Cesare and Lucrezia Borgia). [...] The bumper-to-bumper cars and buses that once roared around Castel Sant’Angelo are now gone. The area around the castle has been turned into a pedestrian zone. Visitors can now walk in peace through the landscaped section with a tree-lined avenue above the Tiber and a formal garden. In 2000, the moat under the ramparts was opened to the public for the first time. You can wander the footpaths and enjoy the new beeches providing shade in the sweltering summer." (Darwin Porter & Danforth Prince: Frommer's Rome, 17th Edition, 2005).