The siege of the Muslim-held city of Algeciras in Andalusia (in mod. Spain) culminated in its capture by Castilian forces under King Alfonso XI in March 1344.
The astounding victory of the combined forces of the kings of Castile and Portugal at the river Salado (October 1340) broke the joint Marinid-Nasrid siege of Tarifa but did not significantly reduce the presence of the Muslim forces in the region. The impetus of that triumph allowed King Alfonso XI of Castile to carry out another successful campaign on his southern frontier in 1341. In the autumn of that year, the king contemplated a bolder goal: the city of Algeciras. Any lasting territorial gain had become dependent on the control of the Strait of Gibraltar, which would prevent the kings of Granada from gaining help from the Marīnid sultans of Morocco. Tarifa, the natural landing base for North African Muslims since 711, had been conquered by Castile in 1292, but Algeciras, a privileged harbor in a bay protected from uncomfortable eastern winds by the Rock of Gibraltar, was still in Moorish hands. Alfonso XI began to besiege the city in August 1342. He had to resort to extraordinary fiscal expedients: the alcabala (a new sales tax), as well as widespread borrowing and conversion of his own personal silver into money in order to meet the expenses of a campaign that lasted twenty-one months. The Castilian troops were augmented by other peninsular forces. The crusading spirit drove King Philip III of Navarre to take part in the siege, and small contingents from as far away as Germany and England (including the earls of Derby and Salisbury) also took part. The Castilian navy had experienced heavy losses in previous years, and Alfonso XI had to resort to unreliable Genoese and Aragonese ships under the nominal command of the Genoese Egidio Bocanegra.
The siege progressively tightened, especially by sea, the only means of supply left to the inhabitants of Algeciras. After the defeat of a Muslim relief army at the river Palmones in December 1343, both Marīnids and Nasrids realized that Algeciras could not be saved. On 22 March 1344 King Yūsuf I of Granada offered to surrender the place on terms that were accepted by Alfonso XI. Five days later, the Castilian king made his entry into the town. The long siege was the last great example of a medieval assault before artillery became widely used in the second half of the fourteenth century.