Riverrun is the ancestral castle of House Tully, the Lords Paramount of the Trident. Riverrun is a strong three-sided castle, although not especially large. The castle is bordered on the north by the Tumblestone and on the south by the Red Fork, while on the west a third side faces a massive man-made ditch. 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. The castle has sandstone walls which rise sheer from the water, its battlements are crenelated and have arrow loops, and its towers command the opposite shores. Riverrun's keep is located inside. Properly garrisoned, Riverrun can hold supplies for men and horses for as long as two years: a garrison of two hundred men is larger than Riverrun requires in most circumstances.