Wiktionary
vb. (context idiomatic English) To imagine visionary projects or schemes; to daydream; to have an idle fancy, a pipe dream or any plan, desire, or idea that is unlikely to be realized.
Usage examples of "build castles in the air".
It had supplied the plains with many of their greatest wizards and witches and, once again, the philosopher might have marvelled that such a four-square people could give the world so many successful magical practitioners, being quite unaware that only those with their feet on rock can build castles in the air.
Science fiction writers build castles in the air, and the fans move into them.
Every step I took confirmed me in my belief, and I began to build castles in the air in relation to my marriage with my pretty little cousin.
Every step I took confirmed me in my belief, and I began already to build castles in the air in relation to my marriage with my pretty little cousin.