Lacking knowledge of PCS's mind, we can't know what she meant specifically by the castles. Any meaning or story that we attach to them are ones we create in our own minds.
However, if we examine the art of the period and Smith's known influences, we find children's story books, the theatre of Ellen Terry and Henry Irving (emphasis on a 19th c view of Shakespeare - see Charles Lamb, etc.), and Arthurian/mythic England, influenced by the Pre-Raphaelites as it enters pop-culture. The structures are the fantasy Camelot or the castles of Macbeth or Hamlet, or the palace of Old King Cole (was a merry old soul) - a world of Kings, Queens, Knights and Pages, representing a mythic world of adventureous tales and quests. The oft-noted Odysseus-theme in the 10 of Pentacles adds the idea of achieving the end of your quest - a coming "home" to one's true self and the finding of one's true "family" - where one is known for who one really is. And, of course, there is (especially in Pentacles) the idea of monetary success, stability and the establishment of a notable lineage - epitomized by a castle (would a factory have served this better?).
From The Diamond Fairy Book, a piece by Charles Robinson, which book also featured the art of PCS:
http://artofnarrative.tumblr.com/image/53544481499
Or, see the work of Howard Pyle in which castles appear in a great many of his backgrounds:
Correction:
http://artofnarrative.tumblr.com/image/2932588138