The Reeds' floating castle that never appears on screen in Game of Thrones is Greywater Watch.
Greywater Watch is the seat of House Reed, the crannogman family ruled by Howland Reed. It stands somewhere in the marshes of the Neck, southwest of Moat Cailin, on a crannog—a man-made island built in the wetlands. Unlike an ordinary fixed castle, Greywater Watch is said to move through the swamps, making it extremely difficult for enemies, and even ravens, to locate.
The television series includes Meera and Jojen Reed, but it does not show their family castle. Their father Howland is also kept mostly off screen, despite his importance to the history of the Stark family and the events surrounding the Tower of Joy. The location is therefore often confused with Moat Cailin, another stronghold in the Neck that does appear in the series.
Greywater Watch's moving nature is more than a visual novelty: it reflects the crannogmen's expertise in concealment, survival, and guerrilla warfare. Its omission removes one of the novels' most distinctive northern settings.