The late 15th century Baia Castle was built by the over ancient Roman ruins thought to be the summer residence of Julius Caesar. On the 29th September 1538 the great eruption, with devastating effects for all the Phlegraean Fieldsi, caused serious damage to the Castle of Baia too. The castle was restored and enlarged by the viceroy don Pedro Alvarez de Toledo.
Today, this cliffside Aragonese castle with sea views is home to the Archaeological Museum of Campi Flegrei.
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.