In the movie Kingdom of Heaven were there Arabs who fought with the crusaders?

Isaw in couple of scenes where Arab-Saracen dressed people fighting with the crusaders. Did this happen in real-crusades too? if so what was the reason? unless they were Christian-Arabs? am talking abt scenes such as where Balian is marking 400 and these Arab guys are doing something outside the castle in Jersulem i.e preparing for Sallahdin to come.