A Viking ring fortress, ring fort or Trelleborg-type fortress is a type of circular fort of a special design, built by the Vikings in the Viking Age. They are also known simply as trelleborgs. All trelleborgs have a strictly circular shape, with roads and gates pointing in the four cardinal directions. These common structures are sometimes partially encircled by advanced ramparts, but these additions are not always circular.

There are a total of seven known Viking ring fortresses at present, located in Denmark and Scania, Sweden. Most of the seven trelleborgs have been dated to the reign of the Harold Bluetooth of Denmark (died 986). The fort in Borgeby has been dated to around 1000 AD, so it is possible that it too, was built by the same king.