The castle must be positioned just far enough from the sea to be completed before the tide reaches it. As the moat is dug by busy spades, the vacated sand forms a growing pile in the middle. Sea water starts to rise into the moat from below. As the waves break gently, closer and closer, the children dig faster to fortify the outer rampart with sodden sand. Even as the defences are being flooded, they work on shaping and firming the castle with hands and feet. The tide soon overwhelms the mini world and smooths the whole thing flat, leaving the children standing ankle-deep in ebb and flow.