Erk Kala is the oldest part of Merv: a near-circular walled mound about 600 metres across, rising 20 metres above the surrounding plain. It was founded under the Achaemenid Persians sometime in the 6th or 5th century BC and served as the citadel of the city through every later phase. The walls — once 30 metres thick at the base — have eroded into smooth red-clay slopes, but the original ring is still clearly visible from the surrounding flatland.
A climb to the top of the ramparts gives the best overview of the wider Merv site: Gyaur Kala spread out below, the Sultan Sanjar dome a couple of kilometres west, the medieval Sultan Kala beyond. Bring sturdy shoes; the eroded slopes are loose underfoot.