Sky castles are created by a 9th level spell cast by a mage who is intimately involved in the building of a castle. They are piloted from a sofa worth at least 1000gp. Normally these are forbidding costs, but since the The Free Imperia is building a vast number of fortresses by magic with the Fortification Initiative, the extra 9th level spell on a given day is already available. For couch supply, Thalion Paurgiliath Arch Duke of Eltán organized the First Annual Open Arda Maroonish Sofa Championship, with winning sofas displayed on the Imperial Arda Progress, Tree Quest, Historical Explorations, and Furniture Exhibition, as well as favored trading status from the West Arda Trading Company, so a great many fantastically valuable sofas were donated by the premier furniture makers across all of Arda.

They are a key component of the Signal Company's semaphore network. They can bridge signal between points over a much greater range than ground towers, and hop signal over towers which are damaged, under repair, or otherwise out of service.

They move at a walking pace, but can continue day and night. They are standardly equipped with modernized Harpaxii mountable on a wide spread of standardized interchangable anchored sockets, allowing them to grapple faster flying ships or creatures. Their decks and land surfaces are standardly rigged with boarding nets made of cable; approaches to come through the nets are carefully protected. Experiments with towing Sky Castles with Spell Jammers are underway.

They serve as troop transports, signal relays, goods warehouses, Spell Jammer ports, and more. The WATCH (West Arda Trading Company Headquarters) is a giant sky castle and reservoir built around a newly-ancient Fey Cherry tree, and supplies the Field Engineering Corps with preformed castle building materials as well as filling moats. It has a tiny Sky Castle serving as a dedicated tender to move materiel between levels. It houses a protective force up to multiple legions in strength, and can dock multiple Spell Jammers at the same time.

WATCH / moat tanker schematic:

The WATCH's creation led to the building of the Skylocks in the Great Inland Waterway Initiative.