Gihon Spring
The ancient city of Jerusalem, being on a mountain, is naturally defensible from almost all sides, but suffers from the drawback that its major source of fresh water, the Gihon Spring, is on the side of the cliff overlooking the Kidron Valley. This presents a major military weakness as the city walls, if high enough to be defensible, must necessarily leave the Gihon spring outside, thus leaving the city without a fresh water supply in case of siege. This weakness had been removed by the Canaanites who built a very strongly fortified tower around the spring and connected it to the city walls on the slope by an additional wall, which carried a well-protected corridor. A Canaanite tunnel already collected the spring water since ca. 1800 BC and led it southwards through the bedrock, but it released the water onto the fields in the Kidron Valley through several openings and ended in an open reservoir. While allowing for important farming, this arrangement also made water available to besieging troops outside the city walls.