The Wall
The Wall is a colossal fortification which stretches for 100 leagues (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 700 feet tall and is made of solid ice. It was supposedly constructed using both magic and mundane means some eight millennia ago, in the aftermath of the Long Night to defend the realm against the White Walkers who dwell in the far north.
The Night's Watch
The Wall is defended and held by the Sworn Brothers of the Night's Watch. Once an honourable institution, the Watch has fallen on hard times by the time of the War of the Five Kings. It has become grievously undermanned, numbering less than a thousand men, an all-time low. While once great glory and honour was to be had in the Watch, in the present day it is often seen as a glorified penal colony.
The current headquarters of the Watch is Castle Black, located in the shadow of the Wall at the northern end of the Kingsroad. Eastwatch-by-the-Sea, located at the eastern end of the Wall, serves as the main port and resupply post for the Night's Watch. The Shadow Tower at the western end of the Wall is the only other manned castle. Among the abandoned castles along the length of the Wall is the Nightfort, the first fortress built on the Wall and the original headquarters of the Night's Watch. The region south of the Wall is called the Gift, bestowed to the Night's Watch by House Stark thousands of years ago to supply the Night's Watch with food and provisions.
In the past, the Night's Watch would send men out every morning to the north side of the Wall to cut down any trees growing within one mile of the structure, so the wildlings couldn't use any natural cover to approach the base of the Wall undetected. With the ranks of the Night's Watch drastically diminishing over the years, many of the areas between the three forts that are still manned have tall trees growing right up to the base of the Wall.