The fortress is Bornholm's oldest defence works. Its builder is unknown, but an account of the Baltic Sea travels of Wulfstan of Hedeby in 890 tells us that Bornholm already had its own king at the time."Gamleborg", Kulturarv.dk. Retrieved 16 October 2012. There is, however, firm evidence that the fortress was in use during the reigns of Harald Bluetooth (940–986) and Canute IV (1080–1086)."Gammelborg og Lillebord", Naturstyrelsen. Retrieved 16 October 2012. Gamleborg was used as refuge during the tenth century against Viking raids. Gamleborg was abandoned in 1150, the occupants moving to Lilleborg, only to the northwest. It is not known why the move was made but it does not appear to have been the result of hostilities. Excavations in the 1950s showed the fortifications originated in the Viking period although there is evidence the site was used as a hideout in the Iron Age. The ruins that can be seen today are mainly the result of reconstruction work completed in about 1100.
