CASTRO URDIALES  , a seaport of northern Spain, in the province of Santander, situated on the bay of Biscay and at the head of a branch railway connected with the Bilbao-Santander line . Pop . (1870) about 3500; (1900) 14,191 . Castro Urdiales is a modern town, although its castle and parish church date from the middle ages . It was destroyed by the French in 1813, but speedily rebuilt and fortified . Its rapid rise in population and prosperity dates from the increased development of iron-mining and railway communication which took place after 1879 . Its chief industries are iron-mining, fishing, and the preservation of fish, especially sardines, in oil . Between 1894 and 1904 the exports of iron ore rose from 277,200 tons to 516,574 tons .