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

House Tully had served House Mudd of Oldstones, but King Tristifer IV Mudd was killed during the coming of the Andals. After the king's death, Ser Axel Tully castle Riverrun at the juncture of the Red Fork with the Tumblestone, land which he had received from an Andal king, Armistead Vance.

The strategic position of Riverrun brought wealth to the Tullys, who guarded the western riverlands from the Kingdom of the Rock. The castle has rarely been surrendered and has never fallen by storm. The Tullys have ruled Riverrun and its rich lands for at least a thousand years.

During Aegon's Conquest a few generations later, Lord Edmyn Tully raised the banners of House Targaryen above Riverrun, beginning the rebellion of the river lords. Following the burning of Harrenhal, King Aegon I Targaryen named Edmyn the first Lord Paramount of the Trident.

In time of danger the sluice gates can be opened to fill a wide moat and leave the castle surrounded on all three sides by water, turning Riverrun into an island and leaving it practically unassailable.