A very beautiful night over Trondheim. View from Kristiansten Festnin. Kristiansten Festning, historically spelled Christiansten) is located on a hill east of the city of Trondheim in Sør-Trøndelag county, Norway. It was built after the city fire of Trondheim in 1681 to protect the city against attack from the east. Construction was finished in 1685. It fulfilled its purpose in 1718 when Swedish forces laid siege against Trondheim. The fortress was decommissioned in 1816 by king Charles XIV John.