Kuusisto Castle was a medieval episcopal castle built in the beginning of the 14th century. It was the main residence of Finnish Catholic bishops until 1522, when the last bishop Arvid Kurki drowned when fleeing from Danish soldiers.
Kuusisto heyday was in the in the beginning of the 15th century, when bishop Maunu Tawast spent lot of time and money to enlarge the castle. The castle was ordered to be demolished during the Protestant Reformation in 1528 by the king Gustav I of Sweden. Stones of the castle were later used in renovations of Turku and Kastelholma castles.
Excavation and reconstruction work on the remaining ruins began in 1891.
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.