St. Lawrence's Church was built around 1125 AD of travertine. During the reformation in 1536 the church was demolished. Only the tower was left, and is today the tower of the city hall. The ruins are preserved 2 metres under the square Stændertorvet. They reveal the beautiful restored red and black clay tile floor and mediaeval fixtures of the church as well as archaelogical finds from the excavations.The ruins can be accessed with a key available at the Tourist Information.
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.