CASTRO URDIALES, a seaport-town of Spain, in the province of Santander, is well known to sailors for the shelter which it affords from storms in the Bay of Biscay. It was destroyed by General Foy in 1813, but has been rebuilt, fortified, and greatly improved. The most remarkable buildings are the castle and the hermitage of Santa Ana. Its fisheries are considerable. Pop. 2936.