Riverrun is the former seat of House Tully, who continue to hold it in defiance of the Iron Throne and its new lawful rulers, House Frey. It is a large castle located in the central-western part of the Riverlands. It sits at the point where the Red Fork of the Trident River is joined by its major tributary, the Tumblestone River flowing out of the west. In times of danger, sluice gates can be opened to flood a channel cut to the west of the castle, turning Riverrun into an island. Its walls rise sheer from the waters and its towers command the opposite shores, making assaulting it almost impossible without huge casualties
Riverrun is the seat of House Tully, the Lord Paramounts of the Trident and the Riverlands. Prior to the War of the Five Kings, Lord Hoster Tully is the Lord of Riverrun. Petyr Baelish recalls being fostered at Riverrun during his childhood and being enamoured by its high falls.
During the War of the Five Kings, a Lannister army under Ser Jaime Lannister lays siege to Riverrun. Northern forces under Robb Stark lure Jaime's men into a trap at the Battle of the Whispering Wood, breaking the siege of Riverrun.
Following the death of Lord Hoster Tully, his kith and kin including Catelyn Stark, Robb Stark, Edmure Tully, and Brynden Tully attend his funeral. Per Tully tradition, he is placed in a funeral boat. His son and heir Edmure makes several attempts to light the funeral boat but misses. His uncle Brynden is forced to complete the ceremony. Riverrun becomes the headquarters and main base of operations for King Robb, the King in the North. After the funeral, the Blackfish comforts Catelyn, who is grieving over the purported loss of her sons Bran and Rickon Stark.
Lord Edmure leads Tully forces into action against the Lannister bannerman Gregor Clegane at the Stone Mill. Lord Edmure succeeds in expelling the Mountain's forces from the Riverlands and capturing two Lannister squires Martyn and Willem Lannister, the nephews of Lord Tywin Lannister. Despite this victory, Robb and Brynden are furious because Edmure's action interfered with Robb's plan to lure the Mountain into a trap. Meanwhile, Talisa Stark, the wife of King Robb, tends to the wounds of Martyn and Willem in Riverrun's dungeons.
Later, Rickard Karstark and several of his followers murder Martin and Willem in their cell to avenge the deaths of his sons. For murdereing two unarmed prisoners, King Robb orders that Rickard's men be hung and that Lord Karstark himself be beheaded. The execution takes place outside the grounds of Riverrun. This has the effect of turning House Karstark against the Stark-Tully alliance. In his private quarters, Robb discusses his predicament with Lady Talisa. Lacking enough men to attack Casterly Rock, Robb decides to repair his strained relationship with House Frey, whom he had earlier promised to marry one of Lord Walder Frey's daughters.
Lord Walder's sons Lothar Frey and Black Walder travel to Riverrun and discuss their father's new demands with King Robb, Lord Edmure, and Lord Brynden. Robb agrees to apologize to Lord Walder Frey, cede Harrenhal and all its lands to the Freys, and that his uncle Edmure marry Walder's daughter Roslin Frey. After some discussion, the Starks and Tullys acquiesce to the Frey's terms for mending their alliance. The Starks and Tullys depart Riverrun and travel to the Twins, the seat of House Frey. Following the wedding, Lord Frey takes Edmure prisoner and massacres the Stark and Tully hosts including Robb, Talisa, and Catelyn.
Following the Red Wedding, Lord Frey reaffirms House Frey's alliance with the Iron Throne and the Lannisters. As a reward for his help in putting down the Northern rebellion, Lord Frey is made Lord of Riverrun and the Freys are given paramountcy over the Riverlands. However, this victory is soured by the revelation that Lord Brynden had escaped the massacre.
During Petyr Baelish's trial at the Vale, Sansa Stark speaks of Baelish's childhood at Riverrun and his enduring love for Lysa Arryn, the late daughter of Hoster Tully. As a result, Petyr is cleared of any involvement in her death, which is regarded as a suicide.
The Freys lose control of Riverrun when Lord Brynden Tully retakes control of the castle using his knowledge of its walls and passages. In an attempt to reconcile with Sansa, Petyr offers this news to her. Acting on this information, Sansa sends her protector Brienne of Tarth and Podrick Payne to Riverrun to seek reinforcements from the Blackfish.
Embarrassed by the loss of Riverrun and other uprisings in the Riverlands, Lord Walder orders his sons Lothar and Black Walder to retake the castle. To force the Blackfish's surrender, he orders them to take the captive Lord Edmure as a hostage. After being expelled from the Kingsguard by King Tommen Baratheon for attacking the Great Sept of Baelor, Jaime is sent to assist the Freys in retaking Riverrun from Lord Brynden. Meanwhile, Lothar and Black Walder manage an incompetent siege of Riverrun. In an attempt to intimidate Brynden, Black Walder threathens to slash Edmure's throat. However, the Blackfish calls their bluff when he fails to carry out his threat.
Jaime and Bronn arrive with 8,000 Lannister men-at-arms and knights and take control of the siege. Bronn orders the Freys to dig trenches and to position trebuchets around the castle. He also gives orders for Edmure to be well treated and fed. Jaime attempts to parley with the Blackfish. However, Brynden is defiant and reveals that he had enough supplies to last for two years. He also boasts that thousands of Lannisters and Freys will die for the hundreds inside the castle.
Brienne and Podrick arrive outside Riverrun and are detained by Lannister horsemen. Brienne manages to convince Jaime to let her negotiate with Lord Brynden. Jaime agrees to allow the Blackfish and his men to evacuate the castle and travel north to help Sansa and her half brother Jon Snow retake Winterfell from Ramsay Bolton. Brienne parleys with Lord Brynden but he is in no position to aid his niece. He refuses to end the siege and insists on fighting for his home. Running out of options, Jaime convinces the captive Edmure to end the siege in return for sparing the garrison and offering to give his son a proper education at Casterly Rock. He also threatens to catapult Edmure's son and destroy Riverrun if the garrison does not capitulate.
Edmure returns to Riverrun that evening and convinces the Tully garrison to lay down their arms and surrender the castle to the Lannisters and Freys. Brynden tries to countermand his nephew's orders but the garrison recognizes Edmure as the rightful Lord of Riverrun. As a result, the Lannisters and Freys are able to peacefully occupy the castle and hang their banners over its walls. Edmure orders that the Blackfish be put in chains and handed over to the Lannisters. However. Brynden manages to help Brienne and Podrick escape in a boat and makes a final stand against the Lannisters and Freys. Jaime takes command of the castle and learns of Brynden's death. He also sees Brienne and Podrick rowing down the Green Fork.
The lush Riverlands lack the natural defenses of some other regions, but the seat of Riverrun itself has a unique barricade. The three-sided castle on the Red Fork of the Trident River can redirect the water's flow to completely encompass its walls, blocking attack. After the Red Wedding Tywin Lannister awarded Riverrun, the longtime seat of House Tully, to Lord Walder Frey for his service to the Crown. The Tullysrecaptured the castle and held on to it until Jaime Lannister arranged a surrender through Edmure Tully.