Adjective
Built of, or in, the clouds; airy; unsubstantial; imaginary.
Source: Webster's dictionarySee! he sinks Without a word; and his ensanguined bier Is vacant in the west, while far and near Behold! each coward shadow eastward shrinks, Thou dost not strive, O sun, nor dost thou cry Amid thy cloud-built streets. Frederick William Faber