Fyrkat is a ring castle at Hobro in Northern Jutland. It is a part of the fortification built by Harald Bluetooth.

Together with Aggersborg at the Limfjord, Nonnebakke at Odense, Trelleborg at Slagelse and Trelleborg at Skaane, Fyrkat was built by Harald Bluetooth. The five ring castles (or Ringborge in Danish) were fortifications that were meant to help protect Denmark against intruders. 

All of the castles share the same structure, since they have been constructed in the same manner. They have a circular earth rampart and tree palisade. At Fyrkat, two of the "streets" have been placed so they create a cross in the middle of the castle.

Just like Nonnebakken, Aggersborg and Trelleborg, Fyrkat lies close to the water. It was easier to protect the Danish borders and prevent invaders when the fortifications were surrounded by waters.