The Twins, sometimes known as The Crossing, is a castle in the Riverlands. It is the seat of House Frey, a vassal house of House Tully of Riverrun. It consists of two near-identical towers and a fortified bridge over the Green Fork of the River Trident.
The Twins represents the only crossing point over the Green Fork for hundreds of miles in either direction. It is a major barrier to travelers and merchants traveling from the North to the western Riverlands. It lies directly on the main route from Winterfell to Riverrun. Moving from one castle to the other while avoiding the Twins requires a lengthy detour hundreds of miles to the south or hazardously traversing the bogs and swamps of the Neck to the north. Travelers heading south who cannot pass southwest over the Twins must pass southeast along the Kingsroad
The Freys' rise began six hundred years before the events of the series with the building of a stone bridge over the Green Fork. It took three generations to build the Twins, but afterwards they quickly grew wealthy by charging passing travelers for the use of the Crossing. The bridge is a massive arch of smooth grey stone, wide enough for two wagons to pass abreast.
Robb Stark needs to cross the river at the Twins to face Tywin Lannister's forces in battle, but his mother warns him that Lord Walder Frey is proud, prickly and unreliable. She negotiates on Robb's behalf and wins an alliance: in return for allowing Robb's forces to cross and for the allegiance of House Frey, Robb and Arya must marry some of House Frey's offspring. Robb must also take Olyvar Frey as his squire, a knighthood being expected in due course. Robb accepts the deal. His army crosses the river and splits into two forces, a diversionary force to face Tywin and a larger force to ride hard and attack Jaime Lannister's forces north of Riverrun.
The Twins are referenced in conversation between Robb Stark and his eventual bride Talisa Maegyr on the topic of Robb's promised betrothal to any Frey girl of his choosing - upon asked why, Robb declares that The Twins is "a very important bridge".
After the funeral of his grandfather, Robb Stark comes to conclusion that drastic action must be taken to force the Lannisters to surrender by taking Casterly Rock. This plan is hindered by the murder of two Lannister children, Martyn and Willem who were Robb's prisoners by his bannerman Rickard Karstark - causing Robb to take Rickard's head as justice for defying his orders. This action left Robb without the support of House Karstark and a large portion of his army, so he sought to redeem his favor with House Frey and Walder Frey by marrying his uncle to a Frey girl. As Robb had defied the oath he had took to marry a Frey girl himself, he was on bad terms with the Twins, but all seemed to be forgiven. The event that occurred became known as the Red Wedding, and revealed the traitorous intentions of House Bolton and Roose Bolton, and the wrath of Lord Walder Frey as he ordered an ambush attack after the bedding ceremony which resulted in the demise of Robb Stark, his wife Talisa Stark and his mother Catelyn Stark, the imprisonment of Edmure Tully and the massacre of some of Robb's bannermen and a large amount of Northern soldiers were killed in the camps outside the Twins.
After Jaime Lannister takes back Riverrun from the Blackfish, he arrives at the Twins, where Lord Walder congratulates him on his victory and goes so far as to compare himself to Jaime, stating that they are both Kingslayers. This irritates Jaime, who informs Walder that the Lannisters would not help him if he failed to keep order in the Riverlands. Later, as Walder is eating, he is assassinated by Arya Stark, who kills him in revenge for her brother's and mother's deaths at the Red Wedding.
Disguised as Walder, Arya assassinates the remaining male members of House Frey, avenging all the Stark forces slaughtered at the Red Wedding, by poisoning wine given to them. She gives a message to Walder's wife Kitty Frey that "Winter came for House Frey", before leaving the Twins.
Seat House Frey, Lord Paramount of the Riverlands. The Frey family spent three generations building their river-spanning fortress, and they've grown rich from the tolls charged to travelers for crossing.