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.
Information about House Frey that reveals spoilers from the books.