In Denmark and Sweden there are 7 such defense structures. One of them can be visited near the town of Hobro. The fortress was built about 980. For tourists this is just a small grassy hill, so this place is more interesting from a historical point of view.
In ancient times the fort looked like this.
One of these traditional long houses can be visited.
In the very center of the fortress, in the grass, we found a big hare. He jumped only a meter away from us and I think we were scared even more than he. So be careful :)
Theoretically, the entrance should be paid, but we have not seen anyone there and just passed. A kilometer from the castle there is also a Viking center (N 56 37.505 E 9 46.882). This is something like an open-air museum and an educational center.
Getting there: parking N 55 06.336 E 10 11.484. There is no bus to the Viking center, but you can get to the city of Hobro (Hobro Busterminal) and then go on foot 2.2 kilometers. Find here the schedule.