Riverrun is the ancestral stronghold of House Tully, lords of the riverlands since the Conquest. The castle is a massive structure of sandstone, triangular in shape, located at a fork of the Tumblestone River. Some have compared the keep to a massive ship. It is possible to enter Riverrun from the Tumblestone, by ways of a waterway downstream from the Wheel Tower, as well as through the main entrance. The castle is bordered on two sides by the Tumblestone and the Red Fork, and the third side fronts on a massive manmade ditch, which is flooded to create a moat when the castle is under siege. Riverrun is crowned by a massive watchtower, which allows defenders in the stronghold to spot enemies approaching for miles. This advantage, combined with the defensive barrier provided by the rivers and moat, makes the castle extremely hard to take.
Riverrun was the location of Robb Stark's elevation to the rank of King in the North. The River Lords of the Trident, who had never been part of the old Northern Kingdom, also proclaimed their support for King Robb, and devoted their armies and castles to his service (with the notable exception of Walder Frey). At the beginning of the War of the Five Kings, Jaime Lannister besieged Riverrun with a massive host, but was defeated and captured by Robb Stark at the Battle of Whispering Wood. His host was later destroyed. When Lord Tywin Lannister later tried to attack into the riverlands again, Edmure Tully drove back his assaults, saving Riverrun from attack. But after the death of King Robb at the Red Wedding, the Lannisters and Freys once again besieged the castle. The siege was disorganized (due to quarrels between the leaders of the Lannister and Frey delegations) and largely ineffective, even though Ser Ryman Frey held Lord Edmure Tully hostage. Ser Brynden Tully the Blackfish was able to defy the besiegers until Jaime Lannister arrived and negotiated a settlement with the captive Lord Edmure, who surrendered the castle rather than have Jaime storm it and butcher his people. Riverrun then passed into the weak hands of Emmon Frey, an ally of House Lannister.