The oldest of Merv's ruins, Erk Kala (a modern name meaning citadel castle), date from the 5th century BCE. Constructed by the Persian Achaemenians, Erk Kala appears to have been the central city of Margush as it was known to the Achaemenians serving as an important administrative and trading centre. It lay at the hub of the spectacular Silk Roads along which trade between the furthest reaches of the Persian empire flourished.
The site is some 12 hectares in size and lies 17 metres below today's surface. Buried under more than 1,500 years of buildings old and new, it is virtually inaccessible to archaeological exploration. Little is therefore known about this enclosure. It is possible that the ruins of an earlier city lie beneath Erk Kala's ruins.