Moat Cailin

Moat Cailin is a ruined collection of towers located on the Neck. It is part of the North and is subject to the rule of House Stark, but has not been permanantly manned for centuries. It is the lynchpin of the defence of the North from any invasion from the south.

In the books
According to myth, Moat Cailin was built well over ten thousand years ago by the Children of the Forest, though the accuracy of this is unclear. The fortress commands the northern end of the causeway which carries the Kingsroad through the bogs and swamps of the Neck, at a point in Westeros where the swamps extend almost from coast to coast. Thus, any large host has to pass the fortress to enter the North. Due to the placement of the three surviving towers around the bottleneck and with no firm ground to deploy siege equipment to the south, a few hundred archers with sufficient ammunition could hold off a much larger army for some time from Moat Cailin.

Also according to myth, the Children attempted to use Moat Cailin to hold back the invading First Men and, when that failed due to the humans' superior numbers, attempted to shatter the Neck and completely seperate the North from the South in the same manner they shattered the Arm of Dorne centuries earlier. However, the Children failed and only succeeded in flooding it, creating bogs and swamps. However, the cataclysm proved the strength of their power and may have proved instrumental in bringing the First Men to agree to the terms of the Pact that ended hostilities between the two races.