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:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.