The phrase "built castles in air" means to daydream or to make plans or ideas that are unrealistic or impossible. The antonyms of this phrase include being practical, realistic, and grounded. When someone is practical, they are focused on what can be achieved rather than what they would like to happen. A realistic person sees the world as it is without any illusions, and they take an objective and logical approach to life. Being grounded means having a firm understanding of reality and keeping both feet on the ground. Instead of building castles in the air, one should focus on making achievable goals and creating a realistic plan to achieve them.