Castle of San Marcos (also Castillo de Alfonso X El Sabio) is a medieval castle located in El Puerto de Santa María, Cádiz, Spain. The castle was erected as a fortified church by King Alfonso X of Castile, toward the year 1264. It was built on the site of a mosque of which the wall of the qibla survives. In the same square, we can also see the palace of Aranibar, of the seventeenth century.