noun
castle in the air; pipe dream; unrealistic plan
Refers to an impractical, fanciful idea or plan that is unlikely to be realized. Often used in critical or dismissive contexts.
彼の計画は所詮空中楼閣に過ぎない。
His plan is nothing more than a castle in the air.
そんな夢物語は空中楼閣だ。
Such a pipe dream is just a fantasy.