The phrase "castles in air" typically means having unrealistic or improbable hopes or dreams. Some antonyms for this phrase might include having practical, attainable goals or living in the present moment rather than dwelling on unrealistic fantasies. Other antonyms could include being down-to-earth, practical, or rational in one's thinking instead of being overly imaginative or fanciful. When one is grounded and focused on realistic goals, they are less likely to fall into the trap of building castles in the air, and more likely to achieve their ambitions through practical and attainable efforts.