The Neck is the name given to the narrowest point of the Westeros continent, where the North meets the northern edge of the Riverlands and the Bite comes closest to Ironman's Bay. It is a region of thick swamps and marshlands and is controlled by the reclusive House Reed from their moving fortress of Greywater Watch. The people of the Neck tend to be a dimunitive but still-human folk who favour tridents and nets in combat and are known as the crannogmen, or more disaparagingly as 'frog-eaters'.
The Neck extends from the Saltspear to the lower reaches of the Green Fork, spanning approximately three hundred miles from Moat Cailin to the Twins. To the east lies the sea and to the west thick forests. The Kingsway extends across the Neck on a raised causeway. Journeys from the South to the North mean spending quite a few days and nights in the midge-infested marshlands, which puts off some travellers or encourages them to travel by sea instead. The causeway is the only viable safe route for armies to travel through the marshes, and forces them to pass under the killing fields of fire from Moat Cailin, creating a very effective barrier that has protected the North from southron invasion for millennia. The only way for an invader to effective bypass Moat Cailin is to win the allegiance of the Reeds who know of other routes through the swamps, and given the Reeds' strong ancestral ties to the Starks, this is unlikely to happen.
According to legend, after the Children of the Forest shattered the Arm of Dorne to stop the invasion of the First Men, they were forced to abandon the southern half of the continent and withdrew to the Neck. They attempted to repeat the feat by breaking the continent in half across the Neck, but they were unable to do so, instead merely flooding low-lying areas of the region to create difficult-to-pass marshes and swamps and creating a bottleneck at the current location of Moat Cailin. Whilst they failed to separate the North from the rest of the landmass, this act may have convinced the First Men to abandon warfare and start negotiations, eventually leading to the Pact that made peace between the two races for many thousands of years.