What allowed the christian crusaders kingdom to remain until 1291 is the fact that, the Muslims were too divided to conquer the kingdom. The Visiogoths from Spain were able to sent reinforcements. Venice and Genoa used the Kingdom as a trade hub. <span />
Large cities should be your answer. when these large cities sprang up they promised more better paying jobs. when a ton of people moved to the cities they got a little baby happy lol
Quran (holy book) is the answer i think