Fyrkat is a former Viking ring castle in Denmark, dating from c. 980 AD. It is located near the town of Hobro, some distance from the present end of the Mariager Fjord in Northern Jutland. The fortress is built on a narrow piece of land, with a stream on one side and swampy area on the other sides. Likely built during the reign of Harald Gormsson or his son Sweyn Forkbeard, the fortress may have served as barracks or as a defensive stronghold. It would have help to enable control of the traffic on the main land-based trading route between Aalborg and Aarhus. Because of its unique architecture and testimony to the strategic power of the House of Knýtlinga, Fyrkat was inscribed on the UNESCO World Heritage List along with four other Viking ring castles in 2023.

A Viking ring fortress, Trelleborg-type fortress, or trelleborg, 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 confirmed 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. In 2023, the five Danish forts were inscribed on the UNESCO World Heritage List because of their unique architecture and testimony to the military power of the Jelling Dynasty.