The combat of Castro Urdiales (6-8 July 1812) was the second of a series of joint Anglo-Spanish successes that weakened the French hold on the coast of northern Spain.
The British contribution was made by the Royal Navy. Sir Home Popham, with a squadron consisting of the ships of the line HMS Venerable and HMS Magnificent (later replaced by the Abercrombie) and the frigates Medusa, Isis, Diadem, Surveillante and Rhin and two battalions of marines, operated along the coast, cooperating with the local Spanish guerrilla bands to launch a series of attacks that kept General Caffarelli's Army of the North off balance.
The first success came at Lequeitio, which fell to Popham and ''El Pastor' on 21-22 June 1812. Popham then captured the unoccupied forts at Bermeo and Plencia, but an attempt to attack the key fort of Guetarai had to abandon after the French appeared in unexpected strength.
On 6 July Popham arrived off Castro Urdiales, where he met up with the guerrilla band of Francisco de Longa. The French sent a column from Laredo to try and lift the siege, but the combined British and Spanish forces defeated them. On the following day the governor of Castro surrendered. He only had 150 men in his garrison, explaining the limited resistance.
Popham captured 20 guns at Castro Urdiales. He decided to leave a garrison of marines in the port, and use it as a temporary base. His next target was Portugalete, a fortified village at the mouth of the Biblao River, which was attacked on 11 July 1812.
Castro Urdiales remained in Anglo-Spanish hands until the late spring of 1813. On 12 May 1813 it was stormed by French forces under General Foy, who had been given the task of re-conquering the north coast, after an earlier attempt had been abandoned because of the lack of a siege train.