Sand art and play
A sand castle is one of many possible constructions made of sand, either for the entertainment value to the maker or for its hacking value. Usually, sand castles and other such constructs are made at the beach, where the two basic building ingredients, sand and water, are available in abundance.
Sand castles are often made by children, for the fun of making it. Other sand constructions that children like to make at the beach, are sand boats and water works, both in an attempt to either stop the oncoming see or channel water leaving the beach.
A variant on the sand castle is the drip castle, made by mixing extra water in with the sand, and dripping this 'cement' from a fist held high. The effect is Gaudi-esque.
Grown-ups sometimes engage in contests making sand sculptures, a genuine art, and also a craft, in which the goal is to create structures that are hard to believe can be made just out of sand and water.
See http://www.sandcastlecentral.com or http://www.teamsandtastic.com for examples of such works. Other vulnerable media are ice and snow, leading to ice sculptures and snow sculptures.
Other sand constructs at the beach are often made by grown-ups too, but for more pragmatic reasons: little hills or holes that are meant to keep the wind out and perhaps create a sense of territory on the public beach.