The popes converted the structure into a castle, beginning in the 14th century; Pope Nicholas III connected the castle to St Peter's Basilica by a covered fortified corridor called the "Passetto di Borgo." The fortress was the refuge of Pope Clement VII from the siege of Charles V's Landsknechte during the Sack of Rome (1527), in which Benvenuto Cellini describes strolling the ramparts and shooting enemy soldiers.