Wize men sumtimes bild air kastles just for the fun ov the thing, it is only the phools who bild them, and then undertake to liv in them.

[Wise men sometimes build air castles just for the fun of the thing, it is only the fools who build them, and then undertake to live in them.]

Josh Billings (1818–1885) American humorist, aphorist [pseud. of Henry Wheeler Shaw]

Josh Billings’ Farmer’s Allminax, 1876-03 “Front Teeth” (1876 ed.)
    (Source)

A variant can be found in Donald Day (comp.), Uncle Sam's Uncle Josh (1953):

Only fools build air castles and attempt to live in them.
 

I can't find that exact text in Billings' actual works (even in his idiosyncratic spelling).

Shaw returned to this theme in his (non-Billings) column "Uncle Esek's Wisdom," in the 1880-09 issue of Scribner's Monthly, Vol. 20, No. 5:

Building air-castles is a harmless business as long as you don’t attempt to live in them.