In this paper, we bring fresh evidence on the city size distribution from a ‘lab’ represented by the region of Bukhara observed in the 9th CE. At that time this region was homogeneous in all respects (technology, amenities, climate, culture, language, religion, etc.) and yet cities had different sizes. We rationalize the city size distribution of this economy in a simple general equilibrium spatial model of which we estimate the parameters using the method of moments. The estimated model predicts very well the 9th century city size distribution. Spatial centrality is the major determinant of city size. The silk road contributes to explain what centrality cannot. We find little evidence of persistence of the urban structure when comparing the 9th and the 21st century. We find instead that centroid of the region has moved towards the economic core of the Uzbek economy.