When Turks conquered Constantinople in 1453, Pope Callistus III urged all Christians to the Crusades. Many Croats, led by Saint John of Capistrano, were part of the army that defeated 150,000 Turks at the Siege of Belgrade in 1456. When Belgrade was conquered by the Turks in 1521 many Croatian writers and diplomats pointed out the dramatic situation by stating that Belgrade was "the bastion of Christianity, the key to Europe and the fortress of the entire Kingdom of Hungary." In the following year, German Parliament in Nuremberg called Croatia Zwingermaurer (Fortress) and the Austrian Archduke Ferdinand of Habsburg said that "chivalrous Christian nation of Croats is standing as a shield in front of Styria, Carinthia and Carniola, and the whole of Central Europe and Western Christendom." At the session, Prince Bernardin Frankopan asked for help, recalling that "Croatia is a shield and door of Christianity". Fran Krsto Frankopan stated on July 1, 1523 in the memorial to the Pope Adrian VI that Croatia is a "bulwark or door of Christianity, and especially bordering countries of Carinthia, Carniola, Istria, Friuli, and Italy". Croatian baroque poet Vladislav Mencetić wrote in 1665: