These are sandcastles.
The brutalist sedimentary architecture of artist and sandcastle maestro Calvin Seibert, conjured from nothing more than sand and water, smoothed and levelled by knife, trowel and hand. Sez he:
I always start at the top and work down, taking great care to keep the horizontals level. I pretty much make things up as I go along, allowing surprises and engineering difficulties to shape the castles.