Moat Cailin is the ancient ruin that guards the causeway through the Neck, the sole overland road between the north and the riverlands; its three surviving towers stand amid bog and quicksand, and an army holding them can bar the way against a host many times its size. When Balon Greyjoy loosed his reavers upon the north, the ironborn seized the fortress and set a garrison there under Ralf Kenning, sealing the north behind them so that no southron army might march up the kingsroad to trouble their conquests. Held from the south, ringed by lizard-lions and Crannogman arrows, the Moat proved a bone in the throat of any who would enter the north by land.
The ruin was not retaken by storm but by treachery and rot. Lord Roose Bolton, named Warden of the North by the Iron Throne after the Red Wedding, needed the causeway open to bring his strength and his Frey allies home. His bastard son Ramsay, newly legitimized as a Bolton, sent Theon Greyjoy, whom he had broken into the creature called Reek, to treat with the starving ironborn within. Sickness, foul water, and despair had already gutted the garrison; on the promise of safe passage back to the Iron Islands they yielded the towers. The promise was false, as all Bolton promises were: the ironborn were seized and flayed alive, their skins hung along the causeway as a warning. With Moat Cailin taken, the road into the north lay open, and Lord Roose led his host and his Frey allies north toward Winterfell to sit as masters of the realm they had betrayed.