Serra do Pilar's military importance first became evidenced during the Peninsular War when it was used by General Arthur Wellesley to launch a surprise attack on the French across the Douro, and retake Porto. The monastery's prominent location acted as the only Liberal stronghold on the south side of the Douro during the Siege of Porto. The monastery was fortified over the course of the 19th century to take advantage of its strategic location. In 1910 the site was declared a National Monument of Portugal. Reconstruction of portions of the monastery that had been destroyed by successive wars began in 1927. In 1947 some of the monastery grounds were converted into a military barracks which remains on the site to this day. Since the late 20th century the monastery and grounds have been open to the public and the church continues to hold Sunday Mass. The monastery was included in the 1996 UNESCO World Heritage Site designation of Porto's historic centre.