Castro Urdiales is a seaport of northern Spain, in the autonomous community of Cantabria, situated on the bay of Biscay. Castro Urdiales is a modern town, although its castle and the Gothic-style parish church of Santa María de la Asunción, date from the Middle Ages. Its chief industries are tourism, fishing, and the preservation of fish, especially sardines and anchovies, in oil. The "Lolin" and "La Castreña" anchovy canning factories serve as a reminder of the town's closeness to this industry and its proximity to the sea.
Tourism has greatly increased in Castro Urdiales in the last thirty years; many people from Bilbao and other parts of the Basque Country and Cantabria as well as Northern Spain in general keep summer homes in the town. Although the number of people registered in the town is around 32,000, the summer population can double or even triple this figure.The town is popular because of its beaches and scenic harbor. The commercial fishing industry is declining and may soon be eradicated completely.