Merv is one of the great archaeological sites of Central Asia and a UNESCO World Heritage Site. The ruins sprawl across a 1,200-hectare plain and are made up of five distinct walled cities, built side by side and on top of each other across two thousand years: Erk Kala (founded around 600 BC), Gyaur Kala (Hellenistic, from Alexander's foundation in 330 BC), Sultan Kala (medieval Seljuk, 11th–12th century), Abdullah Khan Kala (Timurid), and Bairam Ali Khan Kala (Safavid).
At its 12th-century peak, Merv was one of the largest cities in the world — comparable to Constantinople and Baghdad — and the centre of Seljuk Persian culture. The Mongol destruction in 1221 ended that. What remains is the largest unrestored Silk Road site you can visit: mud-brick walls melting back into the desert, the great turquoise dome of the Sultan Sanjar mausoleum visible from kilometres away, and the keep-style fortified Kyz Kalas standing alone in the dust. Allow a full day; bring water, a hat, and a guide who knows the layout.