The Sandcastle by M. P. Robertson
There is nothing in the whole world that Jack likes better than building sandcastles. But however strongly he builds them, he can't stop the sea from stealing them away. Then one night he is awoken by the gulls outside his window, calling him to come down to the beach. To his amazement, the sandcastle he built the day before now towers above his head. Jack marches across the drawbridge, and embarks on a magical adventure filled with jesters and musicians, feasting and dancing, a beautiful girl and a magic shell. M.P. Robertson weaves a wonderful tale of fantasy and excitement, with magical illustrations, capturing the wonder of a little boy's imaginary adventure.