The Old Town in Kostajnica is located on a river island, and is the only such island in Croatia. Its development can be traced back to the 13th century, when the princes of Kostajnica had their seat there. With the disappearance of their kin in the early 14th century, the old town begun frequently changing owners, and with the fall of Bosnia in 1463 it became the major stronghold on the border with the Ottomans. Trough treason, Kostajnica fall into Turkish hands in 1556 and remained under Turkish rule for the following 132 years. The Old Town has a tripartite floor plan. Originally, the town was directly accessible through the bridge over the river or through the square tower ground floor. The ground floor was vaulted by a barrel ceiling, so on the side towards the river one could enter through a separate vehicular and pedestrian gate with a drawbridge, while the opposite inner part of the courtyard could be entered through a semicircular vehicular aisle cantilevered by a stone frame. The sentry post is located on the western side. Once the citizens liberated themselves from Turks, the old defensive wall got enhanced on both the inner and outer sides, and thus transformed into a powerful bulwark, a pentagonal tower got built in the western part of the town, and the southeast corner of the town got a strong battery tower. The abolition of the military frontier in 1871 and the annexation of Bosnia and Herzegovina in 1878 resulted in the Old Town’s loss of its strategic importance. Once the military left, its modest and dilapidated areas got inhabited by the city’s poor. In 1908 the City Hall launched the restoration initiative and executed the first works in order to enable effective maintenance of the Old Town. These efforts were interrupted by World War II, and the works from the 30-ies were unsatisfactorily performed and very limited. Systematic conservation–restoration works begun no sooner than the year 1966. The first conceptual design, drafted by the local government of Kostajnica, was unrealistic. Recognizing the impossibility of realizing the desire of city authorities, repair work started being systematically planned and executed, only to be interrupted by the Croatian War of Independence in 1991. Kostajnica suffered heavy damage in the war, and the works were resumed in 2003, after the re-integration of the island in Una into the Croatian territory. Works continued according to the somewhat altered preliminary design from 1982. Repair work was conducted constructively and on all the structures of the Old Town and the interior of the square and pentagonal towers was turned into an exhibiting space for the permanent collection related to the history of the Old Town.