The Great Wall of China, Isn't It?

The best place to build a sand castle is on titan, the largest moon of saturn. You can be fined for stealing sand from sardinia - even if you take small drink bottle full of sand. A man in brazil has been living in a sand castle for 22 years. He has to constantly water his house to prevent it from collapsing. The strongest sand castle will be 99 part sand to one part water.