In 1152, Tortosa was handed to the Knights Templar, who used it as a military headquarters. They engaged in some major building projects, constructing a castle around 1165 with a large chapel and an elaborate keep, surrounded by thick double concentric walls. The Templars' mission was to protect the city and surrounding lands, some of which had been occupied by Christian settlers, from Muslim attack. Nur ad-Din Zangi captured Tartus from the Crusaders for a brief time before he lost it again.