CASTRO URDIALES, a port of northern Spain, province of Santander, on the Bay of Biscay and terminus of a branch rail way connected with the Bilbao-Santander line. Pop. (193o) 12,418. Castro Urdiales is a modern town, although its castle and parish church, on the rocky peninsula which protects the tiny bay, date from the middle ages, when it was an important fish ing port. Destroyed by the French in 1813, it was quickly re built and increased rapidly in population and prosperity with the development of neighbouring iron-mines and of railway com munication, which took place after 1879. The port is naturally sheltered but a new artificial harbour has lately been constructed. Iron ore is by far the greatest export, but fish, especially sardines in oil, and chacoli wine are also important.