空中樓閣 [－－楼阁]

Jyutping hung1 zung1 lau4 gok3
Pinyin kōng zhōng lóu gé

Definitions (CC-CEDICT)
  1. pavilion in the air (idiom); unrealistic Utopian construction
  2. castles in Spain
  3. imaginary future plans
Definitions (Wiktionary)
  1. phrase, idiomatic
    castle in the air; illusion