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 superfluous.
Once known this, the first step I made is to take a calc file and dispose the necesary data, in columns we have radio, squared radio, area, weight, pressure, in rows data will be show for every height, starting from up to downside.
Data is related in the calc file, so I start with an imaginary weight and a specific pressure (You can try changing value on E3 and F3). When it gets the neccesary area for divide that weight, it calculates the new weight, that will be the value for next