The stones can be secured with 'hidden mortar' on the undersides, but mortar should never be relied on for structural integrity because it will fail eventually due to water wearing it away. There is a lot to the dry stone walling technique but two main aspects are laying stones over seams, never directly on top of each other, to lock them in to a pattern. Also the center of the wall contains hearting which are smaller stones that fill in the core and lock the larger stones into place. Hidden mortar is typically used on the underside of the top stones when they need to stay firmly in place (like on a seat wall).