Stable stones on Allonby beach

Mrs B built this stone tower on the beach. The rule I teach for stability is that the line of action of the weight acting from the centre of mass must pass through the base area. So how does that apply here? Is it that for each extra stone added, its line of action of weight must pass through the bottom stone's base? That appears to be the case here. If you just went for each stone's line of action passing through the base of the one below, you could build a tower leaning out sideways and that couldn't happen. This structure curves back inwards at the top to achieve balance. My second and perhaps better thought is that it depends on moments about a central axis that might well include the centre of mass of the whole structure. Mass x distance left and right of that would have to be equal.