People who use the English idiomatic phrase “castles in the air” are referring to plans or ideas that they feel are naively unrealistic, or have a very low chance of success. This can relate to personal goals and aspirations, or greater ideas about the world as it is and as it should be. In various English-speaking communities, individuals use one range of phrases to talk about these unrealistic aspirations, and another set of labels to talk about the people who express them.
As an idiomatic phrase, building castles in the air represents part of the metaphorical classification of phrases. The speaker or writer is using the visual image of castles, usually massive and spectacular structures built of stone, being assembled up in the sky, as a metaphorical way to talk about something that is completely impossible, or nearly so. A similar phrase, “pie in the sky,” is also metaphorical, but implements a more abstract visual image. English speakers use other synonymous phrases as well, such as the phrase “pipe dream” to describe unrealistic goals or objectives. Alternately, they may also use scenario-based idioms to describe an outcome as extremely unlikely, such as the phrase “that’ll happen when pigs fly,” or "when Hell freezes over."
Those who are accused of harboring ideas of “castles in the air” are often derisively described by skeptics who believe that their ideas are untenable. These individuals are often called “dreamers” to illustrate that their ideas are only “dreams” that will not ultimately come true. They also may be called “star-gazers” or “fantasizers.” Used with varying amounts of sarcasm and intensity, this kind of expression helps skeptics express disbelief. Some synonyms for “dreamer” are more positive, where those who see others as creating “castles in the air” in a positive way might call them “romantics.”
Against the above kinds of expressions, those who are more idealistic about some issues have created their own popular phrases and expressions to champion the idea of fostering idealism. For example, someone who talks about “thinking big” is often encouraging an audience to “dream” of what could be. A refutation to skepticism comes in the form of a popular song, now familiar worldwide, where the singer says “you may say I’m a dreamer, but I’m not the only one” where listeners are asked to “imagine” the world as it could be. This interplay of idiomatic phrases is a poignant illustration of how differently people view the world.