Jerusalem, before being captured by King David, was occupied by a Canaanite tribe called the Jebusites, and the city itself was called Jebus, not Jerusalem. This tribe put up heavy walls around the city and fortified it so that it became impregnable. However, the water of the city was derived from a nearby spring so pipelines were built connecting the city to the spring outside and it is through these pipes that David’s general entered the city and conquered it. This incident is also mentioned in the Book of Samuel and the Book of Chronicles