Clashes and raids on bordering Andalusian lands did not keep the Christian kingdoms from battling among themselves or allying with Muslim kings. Some Muslim kings had Christian-born wives or mothers. Some Christian warriors, like El Cid, were contracted by taifa kings to fight against their neighbours. Indeed, El Cid's first battle experience was gained fighting for a Muslim state against a Christian state. At the Battle of Graus in 1063, he and other Castilians fought on the side of al-Muqtadir, Muslim sultan of Zaragoza, against the forces of Ramiro I of Aragon. There is even an instance of a crusade being declared against another Christian king in Hispania.