Viking ring fortress

A Viking ring fortress, Trelleborg-type fortress, or trelleborg (pl. trelleborgs), is a type of circular fort of a special design, built in Scandinavia during the Viking Age. These fortresses have a strictly circular shape, with roads and gates pointing in the four cardinal directions. Inside the fort, each quadrant has one, in a single case four, square blocks of longhouses, completing the geometric symmetry. There are a total of five known Viking ring fortresses at present, located in Denmark. They have been dated to the reign of Harold Bluetooth of Denmark, with an estimated near contemporary time of construction c. 980. Their exact historical context is subject to debate.