The siege of Lisbon began in June 1147. A fleet of crusaders from England, Flanders, and the Rhineland had stopped at the port of Porto on their way to Jerusalem, and the Portuguese king, Afonso I, urged them to help take the Moorish city first, so they agreed and the walls of Lisbon came under attack. The blockade was hard.
The crusaders built siege engines and a floating bridge to cut the city off by river, while the defenders held out behind strong walls and prayed for relief from al-Andalus. Food ran short inside the city, and disease struck both sides through the hot months. The end came in October.
Lisbon fell on October 25, 1147. The terms promised the defenders safe passage, but the sack that followed broke the agreement, and many inhabitants were killed or driven out. The city passed to Christian rule. A bishop was installed as the crusaders had demanded.
The capture mattered. It gave the young kingdom of Portugal its largest city and a deep-water port, and it pushed the Reconquista frontier south, so Lisbon would grow into the capital of a country that soon reached the seas. The crusaders sailed on, most of them, toward the Levant. The siege showed how a small force could take a walled city with ships and patience, and it became a model for later coastal campaigns in the Reconquista.