Trelleborg is the name of a Danish viking fortress, which was excavated near Slagelse in the years 1934 - 1942. Trelleborg is one of five ring fortresses built during the reign of Harald Bluetooth.
Trelleborg can be traced back to ca. 980 and was the first viking fortress to be found in Denmark. Trelleborg was constructed with a circular rampart, upon which was erected a wooden palisade. Trelleborg had four gates and two thoroughfares which intersected in the middle to form a crossway. Trelleborg has a diameter of 120 metres. 135 gravesites have been located at Trelleborg as well as 16 longhouses.
Along with Fyrkat near Hobro, Aggersborg at the Limfjord, Trelleborg at Trelleborg in Sweden, Borgeby near Lund as well as Nonnebakken near Odense, Trelleborg comprise the list of viking fortresses known as ring fortresses. They were all built in the 10th century during the reign of Harald Bluetooth.
The ring fortresses are important to our understanding of the viking age.