(das) Luftschloss
Noun: (literally: air castle) pipe dream
This idiomatic expression refers to a pipe dream, an unrealistic and impossible dream, which usually cannot be achieved due to issues of impracticality: for instance, having a castle in the sky.
Ich wollte die erste Person auf dem Mond sein, aber das ist nur ein Luftschloss. = I wanted to be the first person on the moon, but that's just a pipe dream.
Neuschwanstein Castle. Photo: Helmut H. Kroiss via Pixabay