Riverrun

Riverrun is the seat of House Tully, which was once occupied by 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.

Background
Riverrun is the seat of House Tully, the Lord Paramounts of the Trident and the Riverlands. It was built as an island fortress by Axel Tully. 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 walls.

In the books
In the A Song of Ice and Fire novels, Riverrun is a triangular castle made of sandstone. It is located at the confluence of the Tumblestone River and the Red Fork of the The Trident River.

Riverrun has been in the possession of House Tully for a thousand years, and it became the seat of rule over the region when House Tully rebelled against the ironborn occupiers of the Riverlands and allied with Aegon the Conqueror three centuries ago.

Prior to the Red Wedding, Robb Stark leaves Brynden Tully holding Riverrun as "Warden of the Southern Marches", overseeing the defenses of the Stark-loyal territories of the Riverlands, and protecting his Queen, Jeyne Westerling (the book's counterpart of Talisa Maegyr), who also remains at Riverrun.

Following the Red Wedding, according to the agreement between Tywin Lannister and the Freys, Riverrun is granted to Emmon Frey, the second son of Lord Walder Frey and husband of Tywin's sister Genna. Ryman Frey leads two thousand spears to take Riverrun, reinforced by Lannister troops led by Ser Daven Lannister, the new Warden of the West, and several Riverlords that bent the knee following the Red Wedding. However, Brynden anticipates the siege. He gathers all the foodstuffs and goods he can within the castle, expels all the unnecessary mouths, and prepares for a long siege.

As in the show, Edmure Tully accepts Jaime Lannister's terms and yields the castle, but he helps his uncle Brynden escape. Emmon Frey takes his place as Lord of Riverrun.