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

一般来说，城堡是巨大的建筑物，需要打下夯实的地基并且使用石头等材料。

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

如果地基打的不够坚实，城堡就会倒塌的。

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，当然可笑至极。

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

这就是为什么这个习语是用来描述那些不切实际的想法，就像梦跟幻觉一样。

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."

当我们觉得某人描述的计划或者行动方案是不太可能完成的，我们会告诉他“你是在空中盖城堡。”

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

一则类似的谚语是“沙堆上垒城堡”。

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

我们都知道，在海滩上，沙堆城堡不可避免地很容易就会被海浪破坏掉。

They are not built to last a long time.

它们不会维持很长时间。

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

所以说当我们对别人说“沙堆上垒城堡”，我们的意思是他们建成的东西通常是坚持不了多久的。