The location of the city was a militarianmilitary secret, and steep mountains provided excellent naturalsnatural defenses. It could be accessed only by bridge across the surrounding river gorge, and these,. These were made of rope or horizontal trunks tree, and could be removed if the city was threatened by invaders. It has a water supply from springs that cannot be blocked easily, and enough land growing food for about four times as many people as ever lived there.