Trelleborg in Slagelse

You're standing at Trelleborg, just west of Slagelse on the island of Zealand. At first glance it's mostly a big, round grassy bank, often with sheep grazing on it. But this is a Viking fortress, and the best preserved of its kind. King Harald Bluetooth had it built around 980. The timber was felled in the autumn of that year and probably put to use the following spring.