The walls of the city of Krk have been preserved almost entirely, while their oldest part dates back into the prehistorical time. The prehistorical city Curicta (Krk) is known to have been fortified with walls. This is witnessed by an inscription from the 1st century b. C. mentioning the restoration of city walls, which indicates that the walls had already existed. Archaeological research of the western part of the wall in the 1960s found drywalled megalithic blocks dating from the 3rd century b. C. Such construction preceded the Roman style of construction known as opus quadratum. Archaeological research at the square Vela Placa and the house Perossi (café Volsonis) discovered a later antique wall with many in-built spolia (stellae, Liburnian cippi, parts of architrave…). The antique walls already had towers, while the Medieval and the Renaissance ones still visible today, constitute an important part of the city’s appearance. Besides the Castle and its towers, the polygonal towers from the 15th century on the coast and the main city gates, i.e. the former tower of the Town Hall on the main city square, the city had at least seven or eight smaller towers. The walls clearly visible today emerged mostly in the 14th and the 15th century, at the time of the emergence and development of fire arms. This is also confirmed by the stylistically defined embrasures and the crenelations (merlature).

The old Frankopan castle is situated on the Square Kamplin, opposite to the Krk cathedral. The castle was the central and final point of the city’s defence. It was constructed in several stages, from the 12th until the 18th century. The oldest architectural element of the castle is the rectangular Frankopan tower, so called court house. We know that it was finished in 1191 owing to the Latin inscription in the lunette above the door. Its manner of construction distinguishes it from other architectural components of the castle, while the masonry of the large dressed stone blocks remind of the nearby Romanesque church of St Quirin. This difference in the way of construction is the basis of the theory about its possible independent existence in the time before the Castle existed in Krk and its function as the cathedral bell tower.
The south-western tower of the Castle, the so called Austrian, is connected by wall to the remains of the tower above the Pizanska Gates. Considering the technique of construction and individual stylistic elements, it was probably built in the 14th century.

The round tower of the Castle, called the Venetian tower, is directed towards the Square Kamplin. There is a stone plaque built into its walls with the inscription Aurea Venetorum Libertati and the symbol of St Marco. The ground floor has visible conical cannon openings. The original entrance to the tower, built with regular dressed stones with an iron grate that came down through the slot, is today walled in. The Frankopan Castle is the only preserved profane Medieval monument in the city of Krk. The Castle was partially reconstructed; there is a promenade on a part of the walls and a small display of archaeological monuments.