Petrusse Casemates origin goes back to 1644, when the Spaniards reinforced the medieval fortifications. Under the supervision of the Swiss fortress builder Isaac von Treybach, they built - among other defence works-the powerful Beck Bastion, named after Governor Baron Johann von Beck, a native of the city who had played a key role in the Wallenstein affair on the side of the Emperor. Initially this bastion was as high as the adjacent terraces on the right; it was raised to the present level of Constitution Square (the wall is 27 meters high) by Vauban in 1685.
In 1673 the Spaniards erected the so-called 'Ravelin du Pate' to strengthen the defence of the Beck Bastion; this triangular construction is one of the few well-kept fortifications. Marshall de Vauban conferred the present shape to all the Petrusse fortifications and built the 'Small Staircase'. From 1728-29 the Austrians added the 'Bourbon Lock' and the 'Large Staircase' and in 1746 the casemates of the 'Petrusse Battery' (54 gun emplacements).
One century passed and the fortress was enlarged and reinforced: the second ring was extended and the third started, so that Luxembourg became the 'Gibraltar of the North'. By and by, the Petrusse fortifications fell into oblivion and neglect, as their strategic momentum limited itself to the valley. After the dismantling, stipulated by the 1867 London Treaty, they confined themselves to walling up the loopholes and most entrances. In 1933 the Petrusse casemates were opened to the public.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.