Frankopan Castle was raised by Croatian noble family of Frankopan through several centuries and several generations. After the death of the first Count (knez) of Krk Dujam I Krčki, his sons continue to dominate over the island and the town of Krk. They left first architectural traces in the town of Krk. The oldest part of fortification is the square tower with the diocese, in which Frankopans courtroom was located. The inscription on the lunette over the entrance confirms that the building was raised in 1191, during the time of Bishop John and counts Vid I and Bartol I (Dujam's sons), with the help of the entire municipality.
The round tower on the northern corner of the castle with a transversely extended lower part, was built after the square, probably in the 13th century. It was restored around 1480, and again around 1600. On the tower there is bricked inscription Aureae Venetorum libertati with lions of St. Mark, which dates from 1500. This cylindrical tower has two floors and attic. On the ground floor once there were two doors, one open to the Kamplin, and the other toward the interior of the castle. On the eastern part of the castle, overlooking the sea there is still a square tower, while on the western part of the castle there was a sentry box for guard. Within the complex there was space for soldiers. It is not known precisely when, but the entire complex was completed before 1348.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.