Also known as Flavióbriga after the Roman settlement which stood here, Castro Urdiales has all the charm of a small seafaring and fishing village with a rich tradition and tourist atmosphere, aided by the beaches of Urdiales, El Fraile and Brazomar.

The town centre, with its characteristic houses with wooden rows of balconies, is medieval in origin and earned recognition as a Historic Site in 1978. Its main monuments, of different styles and periods, contrast with the seafaring sobriety of the old village, formed by narrow streets which are perfect for strolling through. The buildings of the church of Santa María, from the 13th to the 15th centuries, the most significant example of Cantabria's Gothic architecture, and the castle-lighthouse, visible from all parts of town, are the most representative sights in Castro Urdiales. The town also possesses highly valuable examples of cave painting, such as in the cave of Peña del Cuco, which contains drawings from the Upper Paleolithic period, with reproductions of deer, horses and goats. Equally worth a mention is the Roman archaeological site of Flavióbriga, which occupies the entire subsoil of the current city, at a depth of around two metres.