Today, when a person says that someone builds castles in the air, they are usually metaphorically describing someone who dwells in a realm of fantasy, and has a poor grip upon reality. The poem "Mopsus, or The Castle Builder" literalizes the metaphor of castle-building in the air in its tale of a central character with a poor grip upon reality, as well as deals with this phenomenon on a metaphorical level. Mopsus wishes to live in a fantasy castle of his own making, even though he is of humble birth. He is thrown out of the real castles he believes will be his destiny to live in, but always has the security of the castles of his mind, that he dwells of, as a lord, a castle of dreams and darkness. Throughout the poem, Mopsus' gullibility and tendency to build "castles in the air" proves his undoing, as it provokes a pattern of deceit in people who wish to profit from his inability to distinguish what is true and what is fiction. The poem is structured upon contrasts between the real and the unreal, the humble and the high, and darkness and light.
             The irony of Mopsus' birth, the poem begins, is that it proved to be a curse upon his father, rather than a blessing, although Mopsus believes himself deserving of high birth, rather than the actual state of his parentage. Rather than welcoming the son's birth the poet states: "Well for his Sire he had no more than one, [son]" undercutting the traditional fairy-tale like quality of a poem set in "days of yore," in a pastoral setting of the old man's "peaceful Grange," where many sons might be seen as a good thing. Like most aspiring youths of fairytales, Mopsus desires to move " beyond the Prospect of his Father's Grove," but this "Pride" provokes "black Shade[s]" that cloud the heavens when he does so, rather than augurs of good fortune, for this son wishes to escape his destiny through dream...