Brahehus Castle was built by Per Brahe between 1637 and 1650s. Soon after Per Brahe died in 1680 Brahehus was abandoned and moved as a Crown property in the Great Reduction under Charles XI of Sweden. In 1708 the castle was destroyed by fire and not rebuilt anymore. From the ruins you have a fantastic view of lakes Vättern and Visingsö.References:
Kristiansten Fortress was built to protect the city against attack from the east. Construction was finished in 1685. General Johan Caspar von Cicignon, who was chief inspector of kuks fortifications, was responsible for the new town plan of Trondheim after the great fire of 18 April 1681. He also made the plans for the construction of Kristiansten Fortress.
The fortress was built during the period from 1682 to 1684 and strengthened to a complete defence fortification in 1691 by building an advanced post Kristiandsands bastion in the east and in 1695 with the now vanished Møllenberg skanse by the river Nidelven. These fortifications were encircled by a continuous palisade and thereby connected to the fortified city. In 1750 the fortress was modernized with new bastions and casemates to protect against mortar artillery.