Norse sagas mention the 13th-century fortress of Isegran, an islet in the Glomma that became a further line of defence against Sweden in the mid-17th century. The ruins of a stone tower are visible at the eastern end of the island. It's also the site of a small museum on local boat building (from the time when boats were lovingly handcrafted from wood).
By road or on foot, access is from Rv108, about 600m south of Fredrikstad city centre.