Trelleborg, Denmark

Trelleborg (or Trælleborg) west of Slagelse on the Danish island of Zealand, is one of seven Viking ring castles discovered as of 2014. In its day, 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 that at its time 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.