Borgarvirki [ˈpɔrkarˌvɪr̥cɪ] lies between Vesturhóp [ˈvɛstʏrˌhouːp] and Víðidalur [ˈviːðɪˌtaːlʏr̥] in the north of Iceland, and at 177m above sea level it dominates the surrounding region. Made out of basalt strata, it has been used as a fortress. Borgarvirki is a natural phenomenon, altered by humans in earlier centuries. In 1949, Borgarvirki was renovated by workers who installed a granite lintel at the main entrance.

Borgarvirki is a volcanic plug (gosstapi [ˈkɔsːˌtʰaːpɪ]), and the Icelandic sagas mention that in earlier centuries it was used for military purposes. There is a viewing dial inside.

It is accessible to visitors travelling around the peninsula of Vatnsnes.

• List of columnar basalts in Iceland

65°27.94′N 20°35.94′W / 65.46567°N 20.59900°W / 65.46567; -20.59900