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 Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.