sandcastle
A castle made of sand, typically built at the beach. Often used metaphorically for something fragile or fleeting.
The children built a sandcastle on the beach.
His plan collapsed like a sandcastle.
Compound of 砂 (すな, 'sand') + の (possessive particle) + 城 (しろ, 'castle'). The phrase is a straightforward calque of the English 'sandcastle'.