I know I am late, but I chime in for the sake of this good deed itself!
So I think they were bulding one floor, then pile up soil aroun until in is level with the top of the aformentioned floor, then repeat as if they would start building a floor on the ground. In the end just remome surrounding soil and you got yourself a pyramide. Technologically easiest thing that comes to mind. Effort wise - not so much easy at all.