The Ksar — the old walled town on the east side of the wadi — is what most travellers come to see. Founded in the 13th century at the crossing of two trans-Saharan caravan routes, it was a key stopping point for pilgrims heading to Mecca and a centre of scholarship in its own right. Today it is a partly-abandoned warren of stone-and-mud houses, several of them collapsed and many of them slowly disappearing under the sand dunes that are migrating in from the east.
Wandering its narrow lanes at dawn or in the late afternoon, when the light is low and the heat manageable, is one of the most affecting experiences in Mauritania. You will pass the old mosque at the centre, several of the manuscript libraries marked by hand-painted signs, and the ruins of a Almoravid-era well. Take a local guide — alleys can be confusing, the sand makes them shift, and the families who live here will be more welcoming if you have an introduction.