The corridor is formed by Mura Leonine (pictured) built by Leo IV in 848-852 to defend St Peter's. When popes returned from Avignon they were using it as a passage between their residence in the Vatican and Castel S.Angelo. Alexander VI modified it and Urban VIII cleaned the corridor from the old little houses standing here and there. In 1527 Clement VII used it as a rescue from landsknechts during the sack of Rome, running to fortified Castle S.Angelo.