1. Constantinople was barely threatened by the Ottomans, the only reason why it was taken over was because it was sacked (I believe it was the 4th crusade) by Catholic crusaders. 2. The real purpose of the Crusades was to obtain more territory for the Holy Roman Empire in the Middle East, though Pope Urban II stated that it was to "retake" Jerusalem to get people to join up.

Only to invade it for themselves in AD 1204 during the 4th Crusade where prostitutes and horses were brought into our Hagia Sophia, various Uniate churches established to undermine the authority of the Orthodox Church, and a multitude of our relics including the Shroud of Turin were stolen for the Vatican. The Crusades weren't just a series of war between Western Christendom and the imperialistic Islamic menace; they were wars against Orthodox Christendom which was victimized by both sides of the conflict.