Hald Castle was built in 1528 by the bishop Jørgen Friis, who was the last Roman Catholic bishop of Viborg. It is said that Friis was one of the first prisons in Hald tower after the Reformation. The castle was left to decay and in the 1700s it was abandoned. Anyway in late 1800s the owned Christopher Krabbe reconstructed the tower which is today the most visible part of the former castle. Today castle ruins are open to the public for free.
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.