The work was started in 1090 but most of the walls appear to have been rebuilt in the 12th century. The enclosed area is an irregular rectangle of 31 hectares with a perimeter of some 2,516 meters,including 88 semicircular towers. The walls have an average breadth of 3 metres and an average height of 12 metres. The nine gates were completed over several different periods. The Puerta de San Vicente (Gate of St Vincent) and Puerta del Alcazar (Gate of the Fortress) are flanked by twin towers, 20 metres high, linked by a semicircular arch. The apse of the cathedral also forms one of the towers.