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.
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. The town is popular because of its beaches and scenic harbor.
Castro Urdiales was originally called Portus Amanus, and was the chief city of the Autrigones. In AD 74 a Roman colony was established under the name Flaviobriga, during the reign of emperor Vespasian. It was most likely established to mine the abundant iron in the area.
The Church of Santa María de la Asunción is in Gothic style. Built under the protection of King Alfonso VIII of Castile in the 13th century (though it was finished in the 15th century), it is a basilica church with three naves. In the interior are the images of the White Virgin and the Reclining Christ, and three Gothic carvings of the Magi. It was declared a National Monument in 1931.
The Castle of Santa Ana is located near the port and the church of Santa María de la Asunción. In modern times it housed a lighthouse.