From A Wiki of Ice and Fire
The Wall is a massive wall over 700 feet high and is made of ice, stretching 300 miles across the northern border of the Seven Kingdoms, separating it from the wild lands beyond and is regarded as one of the nine Wonders Made by Man. The Wall is defended and held by the Sworn Brothers of the Night's Watch, who patrol and guard the castles from the Frostfangs Mountain range in the west to the Bay of Seals in the east.
The Wall was reputedly created over 8,000 years prior to the present setting by Brandon the Builder after the Long Night to defend the realms of man from the wildlings and Others. Approximately 300 miles in length and 700 feet in height, the Wall is protected by the Night's Watch and according to some, ancient spells and sorcery. Supposedly, Brandon the Builder had placed his foundations along the heights wherever possible. Several mentions in the text suggest that the Wall was much smaller at the time, having been built upon by the Night's Watch builders, who quarried huge blocks of ice from frozen lakes in the haunted forest and dragged them south to make the Wall higher, raising it over the centuries to its present size. However, the severely reduced Night's Watch at the present time is limited to maintenance duty. A total of nineteen fortresses have been built along the Wall though only three, The Shadow Tower, Castle Black and Eastwatch-by-the-sea are occupied at the time of the books.
The Wall stretches for 300 miles and is approximately 700 feet tall, It is made of solid ice and stone, and can be seen for miles round. It appears grey or blue depending on the weather and time of day. The top is wide enough for a dozen mounted knights to ride abreast and it is thicker at the base. According to myth, there are old spells woven into it, to strengthen it and keep creatures of a magical nature such as the Others from passing it. 
The Wall is straight from Castle Black to Eastwatch-by-the-sea, but runs like a snake between Castle Black and the Shadow Tower.
The Wall has no gates anywhere, simply a few tunnels cut through it that are secured by heavy iron bars and massive chains that must be unlocked. They are sealed with ice and stone wherever castles have been abandoned or in times of danger. 
The Night's Watch permits the forest to come no closer than half a mile to the Wall. However, over time the places where abandoned Castles stand, the forest has crept back closer. The top of the wall is studded with catapults and cranes. The galleys of Eastwatch patrol the Bay of Seals in part to catch smugglers who bypass the wall through the sea and trade weapons to the wildlings and to the west the mountains and deep river Gorge, make the terrain impassable for all but small groups of raiders.
The Night's Watch raised nineteen castles to guard the hundred leagues of the Wall, however they had never manned more than seventeen at one time. Over the last three hundred years, as the manpower of the Watch has dropped, most of these castles have been abandoned; "the ghost castles" as some have come to call them. During the event of A Game of Thrones only three castles remain in use.
Before the current phase of the Conflict beyond the wall, only three castles remain in use, though sometimes a larger force will be sent to one of the abandoned castles for a fortnight or a moon's turn as well, which was suggested by the First Ranger, Benjen Stark. The manned castles are:
- Castle Black, The largest maintained castle, located at the northern end of Kingsroad and is held by nearly six hundred Brothers.
- Shadow Tower, Most western outpost of the watch, it's garrisoned by two hundred men of the Night's Watch
- Eastwatch, Port town of the Night's Watch and has the smallest standing garrison of the Night's Watch.
- Greyguard, re-garrisoned by thirty men of the Night's Watch under the command of a Steward from the Shadow Tower.
- Stonedoor, given to the Free Folk led by Soren Shieldbreaker, to settle and garrison.
- Icemark, re-garrisoned with thirty men under the command of Bedwyck, twenty from the Night's Watch and ten of King Stannis' men.
- Nightfort, the largest castle on the wall. It was recently given to King Stannis Baratheon as his seat. It would take a long time to make it fit for habitation again.
- Deep Lake, re-garrisons with Free Folk under the command of of a member of the Night's Watch.
- Queensgate, given to the Wildlings led by Morna White Mask to settle and garrison.
- Oakenshield, given to the Wildling Tormund for his clan and seat of his power.
- Sable Hall, re-garrisoned with Wildlings under the command of of a member of the Night's Watch.
- Long Barrow, re-garrisoned with Wildling Spearwives, under the command of Iron Emmett.
- Greenguard, given to Wildlings of Devyn Sealskinner's clan to settle and garrison.
The Wall provides a tremendous tactical advantage to its defenders, enabling a small group to hold off a much larger force. Defenders are generally beyond the range of bow shot or catapult and in turn rely on archery, catapult and dropped objects to deter an assault. In particular, barrels containing rocks frozen in ice are effective in dispersing a mass charge. 
Attackers must rely on one of two tactics, a direct assault on one of the few well-defended gates, or scaling the Wall in small groups and attacking a gate from the rear. The gates have been designed with features such as murder holes and limited access that make them difficult to force. In an extreme case, gates can be filled with rock and ice, thus becoming as impervious as any other part of the wall.
Scaling the Wall involves similar difficulties, as Jon Snow discovered when he feigned allegiance to the wilding cause and joined an infiltration team. A team of determined climbers can reach the top, but only if undetected by defenders, who can easily knock them off with dropped objects or archery. The Wall is thought to "defend itself", disgorging sheets of its own ice as it did to Jarl on his ascent. 
According to legend, the Wall is vulnerable to the Horn of Joramun.
|“||Almost seven hundred feet high it stood, three times the height of the tallest tower in the stronghold it sheltered. His uncle said the top was wide enough for a dozen armored knights to ride abreast. The gaunt outlines of huge catapults and monstrous wooden cranes stood sentry up there, like the skeletons of great birds, and among them walked men in black as small as ants.||”|
- Jon Snow's first impression of the Wall
|“||"If the Wall should ever fall, all the fires will go out." ||”|
|“||“You know nothing, Jon Snow. This wall is made o’ blood.” ||”|
|“||"The Wall can stop an army, but not a man alone."||”|
Behind the scenes
References and Notes
- ↑ A Storm of Swords, Chapter 56, Bran.
- ↑ 2.0 2.1 2.2 2.3 2.4 A Storm of Swords, Chapter 30, Jon.
- ↑ 3.0 3.1 3.2 A Game of Thrones, Chapter 19, Jon.
- ↑ SSM: The wall (September 09, 2000)
- ↑ A Game of Thrones, Chapter 48, Jon.
- ↑ 6.0 6.1 A Storm of Swords, Chapter 41, Jon.
- ↑ 7.0 7.1 A Game of Thrones, Chapter 21, Tyrion.
- ↑ A Storm of Swords, Chapter 54, Davos.
- ↑ SSM circumventing the Wall
- ↑ A Storm of Swords, Chapter 64, Jon.
- ↑ A Clash of Kings, Chapter 68, Jon.