- Samwell Tarly: "The White Walkers sleep beneath the ice for thousands of years. And when they wake up..."
- Pypar: "And when they wake up... what?"
- Samwell Tarly: "I hope the Wall is high enough."
- — Samwell Tarly talks about the Wall[src]
The Wall is a colossal fortification which stretches for 100 leagues (300 miles) along the northern border of the Seven Kingdoms, separating the realm from the domain of 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, though they are now considered myths by most. The Wall has grown considerably since it was first constructed, as the Night's Watch has spent the years augmenting it with ice blocks and upgraded defenses.
The Wall has been rendered ineffective following the breaching of the Wall and, subsequently, abandoned.
The Night's Watch
The Wall is defended and held by the Sworn Brothers of the Night's Watch. Though they were once a respected order, 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 honor 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.
The Watch employs a variety of defenses in order to defend the Wall. The top of the Wall is dug out in a trench-like manner to give protection from the weather. Several outlets and wooden pillboxes allow archers to fire on enemies below. A special pulley system allows archers to hang over the edge of the Wall to fire straight down on enemies attempting to climb it. Other defenses include wooden ramps for dropping stone, ice, and barrels of burning oil on attackers. The Watch also has ballistae and catapults strong enough to kill giants. Perhaps the most impressive defense mechanism is the "scythe" at Castle Black, a massive, bladed pendulum that runs along the Wall with tremendous force, brutally mauling anyone within its swinging arc and raining ice on those below. Following its release, the scythe can be retracted to prime it for another swing.
The physical nature of the Wall is its greatest defense, regardless of how many men garrison it. The Wall is too thick to simply smash through and its height makes it nearly impossible to storm. Normal archers cannot fire all the way to the top. Considering that the wildlings possess little in the way of advanced weaponry like ballistae, only giants armed with huge bows have any feasible chance at hitting defenders on the top. The Wall can also shed large sheets of ice if disturbed, making scaling it an unpredictable and incredibly dangerous endeavor.
On the ground, there are tunnels for rangers returning from beyond-the-wall expeditions. The tunnel is guarded by an outer 4 inch thick, cold-rolled steel gate, and a thin slotted inner gate.
Scale and unit conversion
Measurements in the Seven Kingdoms in both the books and TV series are given in imperial measurements. The Wall is about 100 leagues or 300 miles (482 kilometers) long, about 700 feet tall (213 meters) and averages 300 feet in width (91 meters).
In "Mhysa", Samwell Tarly says that the Wall is 500 miles long. This appears to be an error, as the books clearly state that it is 300 miles long and the evidence in the TV show (such as the distances given for Winterfell to Torrhen's Square and the faster speed of travel between Winterfell and King's Landing) suggests that Westeros may actually be smaller than in the books, while the Wall being 500 miles long would actually make it substantially larger. The claim that the Wall is 300 miles long is further supported in Game of Thrones: A Telltale Games Series, where Britt Warrick says to Gared Tuttle "a 300-mile wall, and you're still in my way". The game is canon to the show so it is possible that Sam was wrong.
In the books
In the A 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. The top is wide enough for a dozen mounted knights to ride abreast and it is thicker at the base. 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 permanently 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.
Numbered on the map at right, these are names of the other castles. Of these, only the Shadow Tower is still garrisoned at the start of the books (as well as Castle Black and Eastwatch, whose names are written out on this map).
- The Shadow Tower
- Sentinel Stand
- Hoarfrost Hill
- The Nightfort
- Deep Lake
- Sable Hall
- The Long Barrow
- The Torches
All nineteen castles were not built at the same time, many of them centuries apart. Castle Black actually isn't the original headquarters of the Night's Watch. The original headquarters was the Nightfort, the first castle built along the Wall, making it possibly eight thousand years old. The Nightfort is larger than all of the other eighteen castles that were subsequently built along the Wall. As the Night's Watch's numbers dwindled, the large castle was so understaffed that it became difficult to maintain. This ultimately led to the Nightfort being abandoned 200 years ago, and the headquarters of the Night's Watch was moved to Castle Black. It isn't clear exactly how old Castle Black is, but it wasn't purpose-built as a replacement for the Nightfort (the castle of Deep Lake was its replacement). Castle Black may have been built after the Andal Invasion, when there were fewer recruits who worshiped the Old Gods: note that Castle Black doesn't have its own godswood with a heart tree, thus Jon Snow has to swear his oath before a heart tree in the Haunted Forest on the other side of the Wall. Some of the other, older castles along the Wall (such as the Nightfort) do have godswoods.
One of the goals that Jon intends to achieve as the new Lord Commander of the Watch is to restore and garrison as many of the abandoned castles as possible, by both members of the Watch and wildlings. So far, most of those castles have been garrisoned, though badly undermanned.
The Wall runs in a straight line from Eastwatch-by-the-Sea to Castle Black, but the western half between Castle Black and the Shadow Tower curves gently here and there like a snake, because it extends through the rough country of the foothills of the Frostfang Mountains. Technically, the Wall doesn't actually extend "from coast to coast": Eastwatch actually does terminate at the ocean's coast (which is why it serves as a port for the Watch), but the western end terminates at a massive gorge that is still somewhat inland. The Milkwater River carved a massive gorge through the Frostfangs which is practically as steep as the Wall itself, and which extends all the way to the ocean. The gorge is held to be as impassable as the Wall - though small raiding bands with climbing equipment have perilously managed to climb both at times. The gorge is still seen as part of the overall defensive line provided by the Wall. At any rate the inlet formed by the Milkwater as it runs into the gorge is loosely thought of as an extension of the west "coast" of Westeros, and characters still speak of the Wall as running from one side of Westeros to the other. Nonetheless, even when all of the castles of the Wall were fully manned, the western end never provided safe harbor for ocean-going ships the way that Eastwatch does.
According to myth, there are ancient spells and sorcery woven into the Wall, to strengthen it and keep creatures of a magical nature, such as the Others, from passing it (this is the reason Coldhands could not pass through it). When Melisandre stays at Castle Black, she feels that her powers are much stronger than before, stronger even than in Asshai; every word and gesture are more potent, and she can do things that she has never done before.
George R.R. Martin has stated that the Wall was inspired by Hadrian's Wall, a massive fortification built across northern England by the Roman emperor Hadrian. Although significantly smaller than the Wall in Westeros (less than 100 miles long and considerably shorter in height), both structures performed the same function; keeping the northern "barbarians" (i.e. "wildlings) out of the "civilized" southern parts of the region. Both walls also had forts at regular intervals that housed groups of soldiers, and in some cases these soldiers were sent from considerable distances to man the wall, similar to how the Night's Watch draws its recruits from throughout the Seven Kingdoms.
- ↑ HBO viewers guide, season 2 map, Castle Black entry
- ↑ HBO viewers guide, season 1 map, Castle Black entry
- ↑ HBO viewers guide, season 1 map, Deep Lake entry
- ↑ HBO viewers guide, season 1 map, Nightfort entry
- ↑ HBO viewers guide, season 1 map, Gift entry
- ↑ 6.0 6.1 6.2 "The Climb"
- ↑ 7.0 7.1 "The Watchers on the Wall"
- ↑ https://www.sfsite.com/01a/gm95.htm