1. A structure built from sand, typically by children on a beach, often resembling a castle
The children built a sandcastle on the beach.
As crianças construíram um castelo de areia na praia.
2. Something fragile or insubstantial that is easily destroyed; an unrealistic or impractical plan
His business plan is just a sandcastle—it will never work.
Seu plano de negócio é um castelo de areia—nunca funcionará.