The historical centrepiece of Kamplin Square is the Frankopan Castle, the fortified residence of the Dukes of Krk and the only surviving secular medieval monument in the city, architecturally shaped between the 12th and 16th centuries. The oldest component of the castle is the square Frankopan tower, built as a courthouse adjacent to the Bishop’s Palace in 1191, as evidenced by a Latin inscription above the entrance. The inscription records that the tower was constructed collectively by the municipality during the time of Bishop Ivan of Krk and Dukes Bartol and Vid. Initially intended as a standalone bell tower for Krk Cathedral, the structure was later incorporated into the castle’s architecture. The round tower, attributed to Ivan VII Frankopan, features a distinctive slanted base and upright upper section, built around 1476. Another tower, known as the Austrian Tower, was constructed during the 14th and 15th centuries. In the 15th century, these towers were connected by a common wall with a sentry walkway and battlements. In the 16th century, Venice reinforced the castle with thicker interior walls and new cannon openings. On the outer wall of the round tower facing Kamplin Square, a stone plaque bears the inscription Aurea Veneto rum Libertati and the winged lion, symbolising St Mark the Evangelist, the patron saint of Venice.

Today, the Frankopan Castle houses the Krk Interpretive Centre, part of the cultural-tourism route “The Frankopan Pathways.” Visitors can learn about Frankopan heritage, view an exhibition of medieval weapon replicas, and explore a small collection of open-air archaeological monuments, including Liburnian cippi (funerary monuments) from the 1st and 2nd centuries.