The biggest archaeological site in Central Asia, Merv played an important part in the versatile Silk Road exchange for more than 2 000 years. Sultan Kala dates from the 11th cent. Sultan Kala means ‘fortress of the sultans’. It is situated to the west of Giaur Kala. Merv became the capital of the Seljuk State in the 11th and 12th centuries. In this period the city reached its greatest prosperity. The city was surrounded by a fortication and a moat by order of sultan Melik Shah. The walls of the medieval city and the citadel Shahriyar Ark are 12 km long.