San Gregorio is a former church founded in the 9th century and, in the 13th century, it became a Benedictine abbey. In the mid-15th century it was rebuilt to the current appearance under design by Antonio da Cremona. In 1775, after a long period of crisis, the monastery was closed and in 1807, after the Napoleonic occupation of Italy, also the parish church was suppressed. Deconsecrated, it was converted in a mint laboratory and then an art restoration center. It is now closed and not used.
It has a Gothic exterior with an ogival portal, elongated mullioned windows with ogival top, and apse.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.