The next more than 350 years Tržan was owned by the Frankopans, not only as one among the many castles in their property, but as the main seat and stronghold of the family in the whole continental part of Croatia.

The walls, bastions and towers were built of hewn stone (ashlar) in the „fishbone“ style and represented a kind of masterpiece of the contemporary building skill.

Bernardin Frankopan (1453–1529), the only son of Stjepan III, successfully managed the whole of his property further from the Tržan Castle, although there was increasing threat of the Ottoman raids from the already conquered Bosnian territory, east of Modruš County.

This led to decrease of importance of Tržan by the end of the 15th century, and the population of the whole area started to move away more and more from its old places of residence to the other, safer parts of Croatia and neighbouring countries, not willing to live in endangered territory.

Following the negligible Ottoman danger at that time, they presumed that it was not necessary to keep soldiers in Tržan and it was left to its own destiny, becoming a badly damaged castle ruin today.