Schengen Castle dates from 1390 but it was was torn down by the industrialist Jean-Nicolas Collart in 1812. He built a residential manor house in its place and now Schengen Castle is a hotel and conference centre.
All that remained of the medieval building was its round central tower. The castle's most famous visitor was certainly Victor Hugo who visited the Collarts in 1871 and made a sketch of the old tower.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.