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 Freys' rise began six hundred years before the events of the series with the building of a stone bridge over the Green Fork.
Travelers heading south who cannot pass southwest over the Twins must pass southeast along the Kingsroad. 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. It lies directly on the main route from Winterfell to Riverrun.
It is a major barrier to travelers and merchants traveling from the North to the western Riverlands. The Twins represents the only crossing point over the Green Fork for hundreds of miles in either direction. It consists of two near-identical towers and a fortified bridge over the Green Fork of the Trident. The Twins, sometimes known as the Crossing, is a castle and the seat of House Frey, the former Great House of the Riverlands. ' The Freys have held the Crossing for six hundred years, and for six hundred years they've never failed to exact their toll.' ― Catelyn Stark
