What is "gaseborg"

Gåseborg is a prehistoric hill fort in the Lake Mälar district in Sweden. The scientists don't know for certain, but most likely it was built to supervise and control the water fair way below the front, probably sometime during the late iron age(500-1100AD). Several major fair way passed write here and from this place the local man at power could watch over the traffic to Uppsala, Sigtuna, Birka and the western districts of Lake Mälaren.