The idiom "castle in the air" means a hope or plan that is unrealistic or unlikely to happen. It refers to a dream or fantasy that is not grounded in reality, like building a castle in the sky.
You can use this idiom when someone is talking about their plans or dreams that seem impossible or impractical. For example, if someone says "I'm going to quit my job and become a famous actor," you could respond by saying "That sounds like a castle in the air. It's important to have dreams, but it's also important to be realistic about what's possible."
This idiom can also be used to describe a situation where someone is wasting their time or energy on something that is unlikely to happen. For instance, if someone is spending all their money on lottery tickets, you could say "Don't waste your time on that. It's just a castle in the air."
Overall, "castle in the air" is a useful idiom to describe unrealistic plans or dreams that are not based in reality.
• Despite the criticism, the artist continued to pursue his castle in the air of creating controversial and thought-provoking artwork.
• Her plan to quit her job and travel the world without any savings is just a castle in the air.
• He spends all his time daydreaming about winning the lottery, but it's just a castle in the air.
• My dream of owning a beach house is just a castle in the air at the moment.
• She talks about becoming a famous singer, but it's just a castle in the air.
• After years of hard work, he finally achieved his castle in the air of becoming a successful business owner.