Why Castles? That is an excellent question.
Robert Cochrane writes about the castles of Arthurian legend playing a large role in his sense of the Arte Magical. He discusses the Castles and the Two Kings and Two Queens only a little in his letters with Wilson and Gills, but they are mentioned and even explained to a certain extent.
Also, my (Laurelei's) teacher within the Craft talked a little of Castles as a circle-casting system. However, we never had enough instruction in that system to adequately make use of it.
So, as we read through the Cochrane materials together, we addressed this question of Castles head-on. Did they fit our system? Which Castles were they? What and who was in them? What purpose did they serve?
The many castles that are mentioned within Arthurian legend (and beyond, in sources like the Mabinogian, etc) are integral parts of the Quest. They each hold treasures and are kept by wise and sometimes fearsome Masters. They guard the Mysteries, each in their own way. So, yes, we saw them as symbolically linked to the Tradition we were creating.
The following is a simplistic overview of each of the Castles. In time, we hope to have fuller descriptions of each. (As of this writing, we only have two described in their own posts.)
The Glass Castle – Caer Witrin, Glastonbury
Keeper: The Holly King, Janicot, the Goat God
Treasure: Glass Orb
Symbol: an empty blue circle
Location: upon a cloud, northwest
Totems: Goat, Holly, Wren
Castle of Revelry – Valhalla, Hell, Golden Castle
Keeper: Hulda, Freya, Brighid
Treasure: Golden Lantern
Symbol: a yellow circle with a yellow dot at its center
Location: surrounded by a Lake of Fire, northeast
Times: Spring Equinox
Totems: Hare, Goose, Birch
Stone Castle – Caer Bannawg, Four-Cornered Castle, Hillfort
Keeper: The Oak King, Basa-Jaun, Cernunnos, the Stag God
Treasure: Stone Bowl
Symbol: a green circle with an equal armed cross
Location: upon a hill, southeast
Totems: Stag, Robin, Oak
Castle Perilous – Grail Chapel, Binah, Bloody Castle, Silver Castle
Keeper: Silver Queen, Cerridwen, Babalon
Treasure: Silver Chalice
Symbol: a red circle with a horizontal line
Location: surrounded by a Lake of Blood, southwest
Times: Autumn Equinox
Totems: Chicken, Swine, Vine