A completely new network of streets was built, with wide streets in accordance with the Baroque ideal and fortifications around the city centre. The result was a fortress town based on the Continental model, surrounded by fortified ramparts to the south and west, and with two tower fortresses – Munkholmen out on the fjord to the north of the city and Kristiansten, on a hill above the city – as free-standing forts. Kristiansten was completed in 1684.
The fortress was expanded and maintained in connection with the Great Northern War in the early 18th century and up until the dissolution of Norway's union with Denmark in 1814, and it was regarded as a strategically important fortress for Denmark-Norway. Naturally, after the union with Sweden, attacks from land were no longer seen as a threat to the city. The fortress served as a fire watch station, which meant that the fortress saw at least a minimum of maintenance until it eventually came to be regarded solely as a historical monument.
The most dominant part of the fortress is the Donjon, a four-storey tower with gun slits surrounded by jagged ramparts that provided cover, and with munitions rooms and gun emplacements. The modest Commandant's Quarters, which were built at the end of the 18th century, are also situated inside the walls. Right outside the main gate is the Haubitz depot, which was built in 1916. The place where members of the Norwegian resistance were executed during World War II is in the central keep.
Today, Kristiansten Fortress is Norway's best preserved 17th century tower fortress, and the characteristic donjon can be seen from all over Trondheim. While most of the bastions around the city centre are now gone, Kristiansten Fortress has survived and is Norway's most intact tower fortress, despite the King's decision in 1816 that the Fortress was to be closed down and left 'to the ravages of time'. Large parts of the grounds around Kristiansten Fortress are now open to the public. Today, the fortress is used for salutes, for historical dissemination purposes, for cultural events, recreation and walking.
The military engineer corps played a part in developing the city and the fortress in Trondheim up until the beginning of the 19th century, and the military activities and art of fortification were forerunners of what is now the Norwegian University of Science and Technology and Norway's most important centre for education and research in the natural sciences.