The Trelleborg, west of Slagelse on the Danish island of Zealand, is one of seven known Viking ring castles. When built, the fortress was situated on a peninsula that jutted into the swampy area between two rivers. The swamp was connected to the Great Belt by a lake and could be navigated by Viking ships. Trelleborg is believed to have been ordered by King Harald Bluetooth in the year 980 AD and it might have commanded the Great Belt and its sea traffic, between the islands of Zealand and Funen.