Axel Tully

Lord Axel Tully is an unseen character in Game of Thrones. He was the founder of Riverrun.

Biography
Due to the Riverlands' lack of natural defenses, Axel decided to construct the castle Riverrun for House Tully. He raised it where Red Fork and the Tumblestone rivers meet. The castle was built with water to each side. This forced soldiers to cross the river in order to breach the castle. Those wearing armor would drown, and would otherwise be within range of archers placed on the battlements.

Reference

 * "Riverrun (Histories & Lore)"