Long Barrow is a castle on the Wall.
In the books
In the A Song of Ice and Fire novels, Long Barrow is an abandoned castle on the Wall located between Rimegate to west and the Torches to the east.
Mance Rayder tells Jon that he once climbed the Wall near Long Barrow, when he traveled to Winterfell for King Robert Baratheon's visit in the guise of a bard.
Jon plans to re-garrison the castle with men from Eastwatch-by-the-Sea. Rattleshirt is seen near Long Barrow shortly before the Battle of Castle Black, and is captured there by Cotter Pyke.
Following an alliance with the Free Folk, Jon re-garrisons the castle with spearwives under the command of Iron Emmett with Eddison Tollett, which inspires the men of the Night's Watch to call it "Whore's Barrow".