Castro Urdiales, a seaport of northern Spain, in the province of Cantabria, 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.