Castles are usually enormous structures requiring strong foundations and the use of heavy construction materials such as stone.

一般來說，城堡是巨大的建筑物，需要打下夯實的地基并且使用石頭等材料。

If the foundations are not strong enough, the castle could collapse.

如果地基打的不夠堅實，城堡就會倒塌的。

The idea of building a castle in the air－with no foundation at all－is, of course, ridiculous.

在空中建城堡，不打地基，當然可笑至極。

That is why this idiom is used to describe ideas that are not realistic, as if they are dreams or fantasies.

這就是為什么這個習語是用來描述那些不切實際的想法，就像夢跟幻覺一樣。

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."

當我們覺得某人描述的計劃或者行動方案是不太可能完成的，我們會告訴他“你是在空中蓋城堡。”

A similar idiom is "building castles in the sand."

一則類似的諺語是“沙堆上壘城堡”。

We know that, on a beach, sand castles are inevitably and easily destroyed by the tide.

我們都知道，在海灘上，沙堆城堡不可避免地很容易就會被海浪破壞掉。

They are not built to last a long time.

它們不會維持很長時間。

So when we say someone is "building castles in the sand," we mean they are creating something that won't endure very long.

所以說當我們對別人說“沙堆上壘城堡”，我們的意思是他們建成的東西通常是堅持不了多久的。