The Gråborg was a castle on the Swedish island of Öland, which today only exists as a ruin of the largest closed prehistoric fortification.
The non-circular castle was built in stages and received its largest expansion in the 12th century. It had an approximately elliptical shape 210 meters in length and 160 meters wide and was surrounded by a four-meter-high protective wall. The castle had three entrances, one of which was equipped with a tower that was built in the Middle Ages. When the castle was abandoned to decay, is not known. In archaeological excavations inside the building no remains of the building could be found. Due to the size of the facility, it is believed that it was a trading venue. The small finds of the excavations are now kept in the Provincial Museum of Kalmar.