Brick vaulted Undercroft which forms the cistern (Aljibe) inside the quadrant of Silves castle, Silves, Algarve, Portugal. The cistern was a key part of the castle's infrastructure as it made it possible to continue supplying people within the castle with water during times of siege. This cistern which was thought to have been built during the Almohad occupation of the Algarve, still supplied the city of Silves with water until the 1990s. The cistern is estimated to be able to hold 1.3 million litres of water when full. During the height of the Muslim occupation of Silves castle the there was enough water to supply 1200 people with water for a year. Today the cistern is utilized as a display and exhibition area.