Greyguard is one of the abandoned castles of the Night's Watch upon the Wall, standing east of Sentinel Stand and west of Stonedoor. Empty for generations, it has fared worse than most of its sisters, its stones fallen in upon themselves.
A Crumbling Ruin
The castles of the Watch were built without walls, meant to be held by living men rather than dead stone, and stone unmended does not endure the cold of the Wall. Greyguard's keeps and halls have largely collapsed, roofs gone and stairs choked with ice, so that any garrison sent to hold it would first have to build it anew.