The ruins of Merv are comprised of five walled cities dating from the 6th century BC to the 18th century, known as Erk Kala, Gyaur Kala, Sultan Kala, Abdullah Khan Kala and Bairam Ali Khan Kala, grouped into three settlements corresponding to three periods, the ancient, medieval and post-medieval Merv.
In the three consecutive invasions of 1221-2 the Mongols sacked the medieval city of Merv, the ruins now known as Sultan Kala. The city remained occupied, an impoverished shadow of the former eastern Seljuk capital until the Timurids integrated the area into their empire in the late fourteenth century. Shahrukh (1405-47) considered locating his capital here and founded a new settlement, now known as the Abdullah Khan Kala, a kilometer south of Sultan Kala. He instituted a major building program, rebuilt the irrigation system, and is credited with construction of the dam. Construction came to a halt however when Shah Rukh relocated to Samarkand, and Merv did not develop again until the Shaybanis reign (1500-98) under whom the fortification walls were erected. In the post-medieval period Merv remained a provincial center, a small town of less than a square kilometer boasting, however, one of the strongest fortresses of the time. A mosque, madrasa and reservoir were situated in the center of the northern section, with a citadel in the north corner.
A rectangular extension known as Bairam Ali Khan Kala was constructed to the west of Abdullah Khan Kala in the eighteenth century. These two sites were probably in use until the early nineteenth century. No longer occupied by the time of the Russian's arrival in 1885, many of the buildings had been dismantled to provide bricks for new construction.
Herrmann, Georgina. 1999. Monuments of Merv: Traditional Buildings of the Karakum. London: The Society of Antiquaries of London.
Nikitin, A. B. and Zeymal, YE. V. 'Merv'. Dictionary of Art vol. 21, 167