The Wall

The Wall is a colossal fortification which stretches for 300 miles along the northern border of the Seven Kingdoms, defending the realm from the wildlings who live beyond. The Wall is reported to be over 800 feet tall and is made of solid ice. It was reportedly constructed using both magic and mundane means some eight millennia ago to defend the realm against the Others who reportedly dwell in the uppermost north, although the Others are now only considered to be mythical.

The Wall is defended and held by the Sworn Brothers of the Night's Watch. Once an honourable institution, the Watch is now grievously under-strength.

In the books
In the Song of Ice and Fire novels the Wall is about 700 feet tall and is defended by three manned castles, the Shadow Tower at the far western end, Eastwatch-by-the-Sea in the far east and Castle Black in the middle, where the Kingsroad meets the Wall. Castle Black is the Watch's headquarters and primary redoubt. Sixteen additional castles stretch along the base of the Wall, but these are no longer permanantly manned due to the Watch's lack of manpower. The Wall was allegedly built by Brandon the Builder after the War for the Dawn to defend against the return of the Others.