A pleasing imaginative construction entertained incessantly, and to his injury, by the patient, but without the delusion that it is a reality. A waking dream—known to be such by the dreamer—of military or erotic triumphs, of power or grandeur, even of mere popularity, is either monotonously reiterated or elaborated year by year. It becomes the prime consolation, and almost the only pleasure, of the dreamer’s life. Into ‘this invisible riot of the mind, this secret prodigality of being’ he retires whenever the necessities of life set him free. Realities, even such realities as please other men, grow insipid to him. He becomes incapable of all the efforts needed to achieve a happiness not merely notional. The dreamer about limitless wealth will not save sixpence. The imaginary Don Juan will take no pains to make himself ordinarily agreeable to any woman he meets. I call this activity Morbid Castle-building.
The same activity indulged in moderately and briefly as a temporary holiday or recreation, duly subordinated to more effective and outgoing activities. Whether a man would be wiser to live with none of this at all in his life, we need not perhaps discuss, for no one does. Nor does such reverie always end in itself. What we actually do is often what we dreamed of doing. The books we write were once books which, in a day-dream, we pictured ourselves writing—though of course never quite so perfect. I call this Normal Castle-building.
Lewis, C. S.. An Experiment in Criticism (pp. 51-52).