Mari was the 3rd millennium BC royal city-state par excellence. It controlled access between central and southern Mesopotamia and the drier plains of northern Syria and the upper Euphrates/Khabur system. Caravan routes through Mari also brought tin for the bronze industries to the west. Its key position between the confluence of the Khabur and Euphrates and the cliffs further south at Baghuz explain its choice as the site for a new city built by political decision. Mari was first occupied at the beginning of the third millennium (2900 BC). Positioned in an area of limited natural agricultural potential, the center based its foundation not only on its trading position but on a sophisticated irrigation scheme. It was surrounded by a circular rampart and ditch (1.9 km in diameter) through which was dug a canal for the dual purpose of water supply and controlling navigation on the river. The late excavations led to development of canals in Mari, including a 120 km navigation link between the Khabur and Euphrates rivers.