Sverresborg (Norwegian:Sverresborg i Trondheim) or Sverre Sigurdsson's castle (Kong Sverres borg) was a fort and residence built in the medieval city of Nidaros (later Trondheim) by King Sverre Sigurdsson. The fortification was built in support of Sverre Sigurdsson's struggle against his rival King Magnus Erlingsson to claim the throne of Norway. The site now forms part of the Sverresborg Trøndelag Folk Museum, an open-air museum for the region of Trøndelag. Location Defense of the city of Trondheim and the location of Sverresborg was based on three key topographical features: The city is located on a peninsula bordered on the east and south by the river Nidelva and on the north by the Trondheimsfjord The neck connecting the peninsula to the mainland was quite narrow and could be easily fortified A glacially-carved-rock hill just to the south dominates the city and provides an easily fortified site (location of Sverresborg) Heklungs War In 1177, Sverre Sigurdsson, who would later rule as king of Norway from 1184-1202, led the rebel Birkebeiner (Birkebeinerne) forces to Trøndelag where Sverre was hailed as king at Øretinget, the Thing (assembly) for Trøndelag. This was an important symbolic event as new Norwegian kings traditionally had been elected there. After this, the Birkebeins moved south and wintered in Østerdalen. The next spring, the Birkebeins returned to Trøndelag, attacking the city of Nidaros (now Trondheim). Defeated, they fled south until they met and beat the army of reigning King Magnus Erlingsson in Ringerike. Encouraged, the Birkebeins again moved north to Trøndelag and remained in Nidaros during the winter of 1178. During the spring of 1179, Magnus arrived with his army. The two armies met on June 19 in the Battle of Kalvskinnet (Slaget på Kalvskinnet) with the result being a clear victory for Sverre, securing his grip on Trøndelag. Magnus Erlingsson again attacked Trøndelag in the spring of 1180. Sverre held Trøndelag from Nidaros, which was then protected by a wooden palisades extending from the river to the fjord across the neck of the peninsula connecting the town to the mainland. In the Battle of Ilevollene (Slaget på Ilevollene), just outside Nidaros, the Heklungs were again defeated. The two forces met again in 1181 in the Battle of Nordnes (Slaget i Bergen 1181) which resulted in a minor victory for Sverre. But Magnus exploited Sverre’s absence from Trøndelag when in November he raided and burned the Birkebeiner fleet located there.