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 in 1527. Clement's successor, Paul III, built a rich apartment, to ensure that the Pope had an appropriate place to stay in any future siege. Decommissioned in 1901, the castle is now a museum, the Museo Nazionale di Castel Sant'Angelo.