- "Our land protects its own. An outsider will find in the Neck an endless morass of suckholes, quicksands, and green grass that looks solid to the unwary eye but turns to water the instant you tread it."
- ―Meera Reed
The Neck is a swamp and marsh-filled region of Westeros, located where the waters of the Bite, an inlet of the Shivering Sea, and Blazewater Bay, an inlet of the Sunset Sea, draw relatively close to one another, making it the narrowest part of the continent. The Neck is the southernmost part of the North, on the border with the Riverlands. 
The swamplands of the Neck are by far the largest in all of Westeros, making it a rather unique biome. It is the only place on the continent inhabited by a species of crocodilians, known as "Lizard-lions" (apparently so-named because they are reptiles the size of lions).
The Neck is inhabited by a unique offshoot of the First Men known as the crannogmen, whose culture has adapted to their swampy environment. The other modern Northerners are also descended from the First Men, but their crannogmen cousins are distinct in several ways. The crannogmen are short of stature, and survive by hunting the numerous animals who live in the swamps. They build their villages on floating artificial islands made of wood (known as crannogs). Like the rest of the North, however, the crannogmen worship the Old Gods of the Forest.
The rulers of the Neck and its crannogmen are House Reed, staunch vassals of House Stark. The seat of House Reed is Greywater Watch, which controls the interior of the swamps. The Neck is bordered to the south by the parts of the northern Riverlands ruled by House Frey from the Twins - which has led to a longstanding feud between the Reeds and Freys over border disputes.
In ancient times the Neck was ruled by the Marsh King of the crannogmen.
They submitted to House Stark when Rickard Stark, King in the North, defeated the Marsh King and took his daughter as wife.
The crannogmen have a hostile relationship with the inhabitants of the Twins to the south. The Freys and their servants call the crannogmen '"frog-eaters" and are disparaging of their martial skills.
The crannogmen maintained their ancient allegiance to House Stark, since King Rickard Stark defeated the Marsh King and took his daughter as wife, even when contact between them and the outside world had faded away to almost nothing. However, several years before the beginning of Robert's Rebellion, Howland Reed of Greywater Watch made the acquaintance of Eddard Stark. They became fast and loyal friends, and Reed stayed at Eddard's side throughout the war. Howland Reed has since become Lord of Greywater Watch and had two children, Meera and Jojen.
The northern end of the Neck is a strategic choke point controlled by the formidable, but usually empty, ruined castle of Moat Cailin. At this point the swamps are so overgrown and flooded to the east and west that large armies can only pass north or south by narrowly following the path of the Kingsroad (which was built over a more ancient road). Moat Cailin's position completely dominates the path of this only route, making any attempt to invade the North from the southern land route futile.
The Neck presents a formidable tactical obstacle to anyone planning to invade the North and was instrumental in holding off the Andals during their invasion of Westeros six thousand years ago. However, it was not effective against airborne dragons, leading King Torrhen Stark, the last King in the North, to his decision to bend the knee to Aegon the Conqueror during the Targaryen Conquest.
In the books
In the A Song of Ice and Fire novels, the Neck was originally a much more pleasant location, but during the war between the Children of the Forest and the First Men it became a point of no retreat for the Children. When their formidable fortress of Moat Cailin looked like it might fall, the Children called upon the same sorcery they had used to shatter the Arm of Dorne centuries earlier to destroy the Neck and split Westeros in half. The sorcery was not effective and they merely flooded the Neck from coast to coast, creating bogs and swamps.
Bear Island · Last Hearth · Karhold · The Dreadfort · Winterfell (Winter town) · Deepwood Motte · Torrhen's Square · Castle Cerwyn · Hornwood · Ramsgate · Widow's Watch · Barrowton · White Harbor · Oldcastle · Moat Cailin · Flint's Finger · Greywater Watch · Rillwater Crossing · Ironrath · Highpoint
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Seagard · Wendish Town · Fairmarket · The Twins · Harrenhal · Raventree Hall · Stone Hedge · Crossroads Inn · High Heart · Acorn Hall · Maidenpool · Pinkmaiden · Stoney Sept · Pennytree · Saltpans · Lord Harroway's Town
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