Siehe Wörterbuch: Luftschloss

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Was ist denn aber das am häufigsten angewendete Wort dafuer?

There are three different expressions because different expressions are used in different contexts. If you supply the context we can say which one fits best.

just wanted to throw this out as another possible translation. haven't seen it in the other threads for this term. I have never heard "castles in the air" in English. I think "pipedream" and "pie in the sky" are synonmous for me.

http://idioms.thefreedictionary.com/pie+in+the+sky

From the Don Maclean song of the same name (perhapts it's an age thing!).
On the other hand, it's not the term I would immediately think of, either.

"castles in the air" is the first thought for me; has it become rare - or is it (mainly) BE? Nick and Spike?

Just a personal feeling: it's not common (any more?) in BE. I would never use it. The Don Maclean reference sprang to mind, I think, precisely because it isn't a common expression in my experience.

FWIW a Google search: "castles in the air" site:UK - and restricted to results during the last year suggests that most of them have not yet fallen from the sky. :-)

https://www.google.co.uk/?gws_rd=ssl#q=%22cas...