- "As long as the ironborn hold Moat Cailin, our armies are trapped south of the Neck."
- ―Roose Bolton
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 permanently manned for centuries. Subsequently, it is neither a fief nor a residence of any lord, but is still the lynch-pin of the defense of the North from any invasion from the south. It is an ancient stronghold of the First Men. It has degraded over time and only three towers still stand. The towers are arranged in mutually defensive positions, suggesting the heightened tactical awareness of the builders.
Moat Cailin was constructed by the First Men during the Dawn Age, and, despite falling to disrepair, has since then remained an important strategic position in defending the North from invasions from the south, most notably being instumental in preventing the Andals from conquering the North during their invasion of Westeros.
King Renly Baratheon discusses a potential alliance with King in the North Robb Stark by negotiating with Robb's mother Catelyn Stark. He offers to recognize Robb's dominion over everything north of Moat Cailin.
With majority of House Bolton's army being trapped south of the Neck due to the ironborn occupation of the castle, Roose Bolton orders his bastard son, Ramsay Snow, to use Reek to take the castle. Ramsay has Reek use his former identity as Theon Greyjoy to convince ironborn garrison to surrender Moat Cailin, in exchange for safe passage back to the Iron Islands. Upon surrendering, Ramsay Snow reneges on the agreement and has all of the ironborn flayed alive.
Sansa Stark and Petyr Baelish pass through Moat Cailin while traveling to Winterfell. Brienne of Tarth and Podrick Payne follow them, but are forced to go all the way around the Moat and the swamp instead of through it.
Behind the scenes
- Renly pronounces the name "Moat Cait-lin" the one time he mentions it in Season 2, but the Boltons pronounce it correctly as "Cay-lin" in Season 4. While it could be a mere oversight that Renly pronounced it incorrectly it could also be seen as a sign of his ignorance of Westeros geography and his cavalier attitude toward being King.
- Renly's offer would've involved the Starks losing a substantial amount of their pre-war territory on the Neck, including Greywater Watch, but Catelyn does not mention this. Possibly it was a script error. It may also be possible that Renly has only a simplified knowledge of geography of the North, or that he was speaking vaguely and considered "the North" to symbolically start at Moat Cailin, because of the strategical importance of the fortress, and meant leaving the North's territory as it was. Or it is possible Renly knew what he was talking about and the "price" for his granting the North its independence was losing the territory south of Moat Cailin. 
In the books
In the A Song of Ice and Fire novels, Moat Cailin is said to have been 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. Moat Cailin was one of the vital reasons why the First Men were able to successfully resist the Andals' attempts to invade the North as they did the rest of Westeros to the south.
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 separate 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.
While in the present era most of Moat Cailin's former towers have fallen into ruin, even the three remaining towers are more than capable of defending the passage to the North, provided that they are fully manned. A key point is that Moat Cailin was only designed to resist attack from the south, and thus its northern flank is relatively exposed to attack by even a small force.
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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