The Twins are a heavily fortified set of castles connected by a stone arch bridge on the Green Fork river. This bridge is wide enough for two wagons to cross abreast and guarded by a tower in the middle known as the Water Tower. The Twins have been the seat of House Frey for over six hundred years and they have grown wealthy by charging a heavy toll on all those who need to cross, as it is the only place to do so within several days travel. As the Freys are both wealthy and numerous they are one of the most powerful houses sworn to House Tully, able to raise four thousand men-at-arms. Still, the Freys have been of suspect loyalty in the past and the current head of the House, Lord Walder Frey, is known as a prickly and prideful old man.
During the War of the Five Kings, the Freys rose in rebellion against the Iron throne, for the King in the North. The rebellion was contingent on a Frey’s betrothal to Robb Stark. However, after Robb Stark broke his betrothal to a Frey for a Westerling, Lord Walder Frey plotted his revenge for the slight. Lord Frey arranged another marriage between Lord Edmure Tully and his daughter, Lady Roslin Frey. Conspiring with Lord Roose Bolton of the Dreadfort and Lord Tywin Lannister, Lord Frey smuggled in a host of sellswords and knights disguised as musicians. During the wedding, the “musicians” began to kill the household and supporters of the King in the North, including his mother, Lady Catelyn Stark, and took Edmure Tully, Lord of Riverrun captive. This event became known as the Red Wedding.