Castle Blackspire, also known as The Castle at the End of the World, is the prison built by Calypso as the prison for the creature created by the Old Gods. It was used by the Dwarves as inspiration for the design of Castle Whitespire.
In the books
Castle Blackspire appeared when Janet, Josh, Eliot, and Poppy were looking for the God of Fillory, Umber. They found the castle by jumping into a lake in Fillory, that reversed the world as it was. When in contact with the God, Umber explained to them the story of Rupert Chatwin, in his point of view, and why he was not the evil twin of Ember, as everyone thought, but just a scared god in exile.
In the TV series
Castle Blackspire is located in Fillory but was flipped to the planet's underside by Ember and Umber; a new castle, Castle Whitespire, was built on top of Fillory instead. The castle greatly resembles Castle Whitespire, but instead features a much darker aesthetic. Castle Blackspire was built by "The Architect", a Nymph named Calypso. Calypso's lover, the god Prometheus, tasked her to build a prison to hold the mistakes that Gods created before humans. Castle Blackspire also housed a fountain which serves as a backdoor to Magic in case the Wellspring was ever shut down by the Old Gods. The fountain can be activated through the use of the Seven Golden Keys, created by Prometheus and imbued with his powerful magic.