Isobaric parametric sand castle

How should we calculate a teoric sand castle internal efforts, and in consecuence, where would it collapse?
This answer, thinking it mathematly, should be the height where the castle has the highest pressure value, witch is calculated as weight over area.
Once we know this, it is easy to think about a mathematic designed castle, that has the same pressure for any height, optimizing that way its shape, and corresponding to gaudi's idea of beautifulness, when an structure has anything superf