Trelleborg is the Viking Age fortress near Slagelse in Denmark known for its precise circular layout.
Built during the reign of Harald Bluetooth in the late tenth century, Trelleborg was one of Denmark’s geometric ring fortresses. Its circular earthwork was divided by roads arranged at right angles, creating a highly ordered internal plan. The fortress also contained longhouses and defensive structures.
Trelleborg was not an isolated curiosity. It belongs to a group of large Danish ring fortresses that includes Aggersborg, Fyrkat and the later-discovered Borgring. Their regular designs and construction on a substantial scale suggest central organisation, probably connected with royal power and military administration.
The name can cause confusion because “Trelleborg” is also used for another Viking fortress in southern Sweden and for the modern Danish museum site. In this question, the specific site is Trelleborg near Slagelse on Zealand, rather than the similarly named Swedish location.