Castles are usually enormous structures requiring strong foundations and the use of heavy construction materials such as stone. If the foundations are not strong enough, the castle could collapse. The idea of building a castle in the air－with no foundation at all－is, of course, ridiculous. That is why this idiom is used to describe ideas that are not realistic, as if they are dreams or fantasies. So when someone describes a plan or course of action that we believe may be impossible to achieve, we tell him, "You're building castles in the air."
城堡通常是一个很大的建筑，需要有坚实的基础，还要用到大量石头之类的建筑材料PIc^kxVNRRWBE*,J;。如果城堡的基础不够牢固，很可能会坍塌yjcQd;Yk[sY+I]J2al7。建立在空中的城堡是没有基础的bNmpWH*@aLx]xU*H[。因此，“在空中建城堡”当然是一个很荒谬的想法f6Eu^|J=fRYx+z。因此，我们可以使用这个成语形容那些不太现实的想法——因为这种想法往往是梦想或幻想罢了JNj]g&ykAgp,8。所以，当有人描述一个计划或行动时，我们觉得那是不可能实现的，就可以告诉他：“你的想法就如空中楼阁一样o+d*~4+wlyiRsLZ。”
A similar idiom is "building castles in the sand." We know that, on a beach, sand castles are inevitably and easily destroyed by the tide. They are not built to last a long time. So when we say someone is "building castles in the sand," we mean they are creating something that won't endure very long.

类似的一个成语是“在沙滩上建城堡Dc*XtSMwjf#SvMyt。”我们知道，沙滩上的城堡不可避免地而且很容易被海浪摧毁，不可能持久pwFeiasBPPCZ]。所以，我们说有人是“在沙滩上建城堡”，意思就是他们正在创造的东西持续不了多长时间]*#vcy3&u;,#GdnVQp。

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