The castle was first built after Ember and Umber flipped over a a distant, wild little world to create Fillory. The Dwarves that built it had based it on Castle Blackspire, a design of Calypso's, which they thought was the most perfect design that they'd ever seen. It took many years to finish seeing as the Northern Spire had not been built just decades before the Chatwins had arrived. 
The castle's sole purpose was to be the seat of power in Fillory for its Kings and Queens to rule by different generations of "Children of Earth". In their periodic absence, the Pickwicks serve as its stewards.
300 years into the future, Fen and Josh have been overthrown and the dark king reigns. Former high kings Eliot and Margo arrive, not realising that they are now in the future.
- The castle is partially run by magic with rotating spires that float above its towers. When magic is disrupted such as, through the Wellspring or Ember, these spires fall off.