High Quality Content by WIKIPEDIA articles! Xabier Castillo Aranburu (born March 29, 1986) is a Spanish footballer currently playing for Athletic Bilbao, as a left defender. Born in Durango, Biscay, Castillo came up through the ranks at Athletic Bilbao, but made his senior debuts with Basque neighbours Real Sociedad, where he had finished his football grooming. His first professional experience came on loan, for UD Las Palmas, in 2006 07's second division. During his first two campaigns with Real Sociedad's main squad, he was an undisputed first-choice, also in level two. On 16 July 2009, Castillo returned his first alma mater, signing a four-year deal.