These medieval fortifications are the most complete and best preserved in all the country.
The work was started in 1090 but most of the defensive wall appears to have been rebuilt in the 12th century.
The enclosed area is an irregular rectangle of 31 hectares (77 acres) with a perimeter of some 2,516 metres (8,255 ft), including 88 semicircular towers.
The walls have an average width of 3 metres (9.8 ft) and an average height of 12 metres (39 ft). The nine gates were completed over several different periods.