Riverrun is the seat of House Tully. 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.