The Dilmun Burial Mounds, built between 2200 and 1750 BCE, span over 21 archaeological sites in the western part of the island. Six of these sites are burial mound fields consisting of a few dozen to several thousand tumuli. In all there are about 11,774 burial mounds, originally in the form of cylindrical low towers. The other 15 sites include 17 royal mounds, constructed as two-storey sepulchral towers. The burial mounds are evidence of the Early Dilmun civilization, around the 2nd millennium BCE, during which Bahrain became a trade hub whose prosperity enabled the inhabitants to develop an elaborate burial tradition applicable to the entire population. These tombs illustrate globally unique characteristics, not only in terms of their number, density and scale, but also in terms of details such as burial chambers equipped with alcoves.