The Twins are a pair of matched, strong castles of great importance as they contain and guard a bridge across the Green Fork near the Neck; there are no other crossings until far further south, at the ruby ford. The Freys were granted the lands some four hundred years before and have since become the strongest houses under the Tullys.
Lord Walder Frey, a very old man with a prolific amount of children and grandchildren, is styled Lord of the Crossing. He has made marriage ties to houses great and small throughout the Seven Kingdoms, and has made preparations to wed his eighth wife, Lady Joyeuse of House Erenford.
Lord Walder has recently joined with King Robb, making marriage contracts between King Robb and any one of his daughters and Princess Arya and his young son Elmar, as well as marrying a daughter, Walda, to Lord Bolton in a separate and unexpected arrangement. Lord Walder’s eldest son and heir, Ser Stevron, rode with King Robb and died fighting at Oxcross. Ser Stevron’s eldest son, Ser Ryman, is now heir to the Twins.
Two grandsons of Lord Walder, both also named Walder, were sent to Winterfell to be fostered. When the Bastard of Bolton razed Winterfell and slaughtered the inhabitants, he shouted for the two Frey boys to be brought to him and they have since written a letter from the Dreadfort.
With King Robb breaking his promise to marry a Frey daughter, the Freys broke off the arranged marriage between Princess Arya and young Elmar Frey, and turned their allegiance although they revealed it only at the Red Wedding, when Roslin Frey was wedded and bedded by Edmure Tully. When that was done, King Robb, Lady Catelyn, and many others were killed while Lord Edmure was taken captive. Before this point, Lord Roose Bolton was married to Fat Walda Frey, which likely helped to seal their traitorous alliance against the Starks. Ser Emmon Frey has been granted Riverrun for the services of the family to the crown.