Moat Cailin is an ancient castle in The North raised by the First Men to protect The Neck. It commands the Neck Causeway, which is the only mapped route through the Neck. It is supposedly never been taken from the south.
Moat Cailin was raised roughly 10,000 years before by the First Men. It had 20 towers and a great blanket wall, almost as large as the one at Winterfell.
Legend says that Moat Cailin was used by the Children of the Forest when they tried to break the bridge between the North and the rest of Westeros, but they failed and just flooded the land, instead creating the marshes and bogs of the Neck.
Moat Cailin was part of the land ruled by the Marsh Kings before House Stark took over the North. They would ally with the Barrow Kings, Red Kings and the Kings of Winter against the Andal Invasion, and would hold Moat Cailin as a final line of defence. The castle wouldn't fall and that was an important reason for the success of the Northmen repelling the Andals.
A Game of ThronesEdit
Robb Stark stops at Moat Cailin on his journey south to liberate his father, Eddard Stark, from captivity in King's Landing. He takes the Gatehouse Tower as his seat, whilst House Umber get the Children's Tower and House Karstark get the Drunkard's Tower. he leaves Lord Howland Reed behind to hold the Neck and Moat Cailin.
A Clash of KingsEdit
After launching his invasion of the North Balon Greyjoy sends his brother, Victarion Greyjoy, to take Moat Cailin by sailing up the nearby river. Balon thinks that, by holding Moat Cailin, he can pull Robb Stark into a trap.
A Storm of SwordsEdit
Robb tries to use Balon's death as an opportunity to strike at Moat Cailin and retake it, as he knows that Victarion will have left to try and secure the crown of The Iron Islands. He is killed at the Red Wedding before he has the chance.
A Dance with DragonsEdit
Ramsay Bolton decides to send Theon Greyjoy into Moat Cailin so that he can lift the ongoing siege and Roose Bolton can return to the North. Theon makes his way into the castle, but has to kill the castellan, Ralf Kenning, who had been poisoned by a crannogman. Theon offers the men safe passage if they agree to stand down. Dagon Codd is killed, as he opposes the idea, but the garrison eventually surrenders. Despite the promise of passage all the prisoners are flayed before Roose arrives from the south.