The Nightfort

The Nightfort is a ruined castle on the Wall, west of Castle Black. It was abandoned due to the high cost of its upkeep. It is reportedly the oldest castle on the Wall, predating Castle Black by centuries, if not millennia. It is not manned by the sworn brothers at the time of the series. It is ill-omened, but some stories say it holds a great secret of the Watch.

According to legend, the Nightfort served as the seat for the Night's King, who had been seduced by a woman from beyond the Wall. He brought her back to the Nightfort and performed dark sacrifices to the White Walkers. However, he was defeated by the King in the North and the King-beyond-the-Wall, Joramun, who joined forces to destroy him.

In the books
In the A Song of Ice and Fire novels, the Nightfort is the oldest and largest castle on the Wall, and was likely the first major stronghold of the Night's Watch after its founding, which according to myth was almost eight thousand years ago.

In one of the Nightfort's stories, an Andal King visited the Nightfort and insulted the cook of the castle. The cook, furious, killed the King's son and served his flesh to his father in a pie. The gods, angry at this violation of the laws of hospitality, transformed the cook into a massive rat that would only feed on its offspring. In another story, a young girl disguises herself as a boy called Danny Flint to serve on the Wall, but is discovered, raped and murdered by persons unknown. In other stories, seventy-nine sentinels stood guard at the Nightfort, and it is where King Sherrit called down a curse on the Andals. It is where a ranger called "Mad Axe" went insane and killed several of his sworn brothers. Symeon Star-Eyes saw hellhounds fight at the Nightfort in the Age of Heroes. Finally, three centuries before the Targaryen invasion, the commanders of the garrisons at the Nightfort and Snowgate went to war against one another, but they were both executed by the Starks for betraying their vows.

The Nightfort remained occupied up until two centuries ago. By this time the Nightfort had become mostly unoccupied and fallen into disrepair, with most of the administrative functions of the Watch transferred to the more centrally-located Castle Black to its east. When Queen Alysanne Targaryen visited the Wall on her dragon, Silverwing, she convinced the Watch that the Nightfort had to be abandoned. She convinced her husband, King Jaehaerys I, to fund a new, smaller castle to be built at Deep Lake a few miles to the east, and the Nightfort was abandoned. By the time of the books, Deep Lake itself has also been abandoned.