H is plan was only a castle in the air.

他的计划只是空中楼阁而己，掌上英语学习机。

Their plan is nothing but a castle in the air.

他们的计划只不过是空中楼阁。

Nobody is safer than he who lives in a castle in the air.

生活在空气城堡中的人是最安全的。

Agricultural production is assisted is more a castle in the air.

农业生产辅助更是“空中楼阁”.

Heart on the road, feet not on the road, the front is always a mirage, castle in the air.

心上路，脚不上路，前方永远是海市蜃楼、空中楼阁。