These forts were a place of refuge in case of danger, as well as a setting for ceremonies and gatherings. Some were still used in the Middle Ages. Ismanstorp (c 400) has a diameter of 125m; ramparts (3 to 4m high) with nine doors encircle the foundations of 88 houses. Gråborg is the largest fort on the island (c 450); its ramparts (up to 6m high and 11m thick) form a circle with a circumference of 640m and a diametre of 200m. Eketorp (museum) contains remains of three successive colonies established between 300 and 1300.