The Trelleborg near Slagelse on the Danish island of Zealand is one of the Viking ring castles of which five or six have been found until now (2006). It lies to the northwest of Slagelse. At the time the fortress was built it was situated on a peninsula that jutted into the swampy area between two rivers. It was connected to the Great Belt by a lake that at its time was usable for Viking ships, but that has since vanished. It might have commanded the connection to the island of Fyn and the sea traffic on the Great Belt.