Fortress Kastel is the oldest historical monument in the City of Banja Luka. The oldest traces of settlements on the territory of Banja Luka are the remnants of Neolithic settlement that were exactly found on the territory of the city fortress Kastel. It is found in the central part of the city, which dominates over the left bank of the river Vrbas. In the past Kastel was a strong army fortification and it protected the basin of the river Vrbas from enemy rush. The fortress is surrounded by thick stone walls on all sides, and in its inner part, apart from the summer stage, playground for children and national restaurant, there is the Institute for protection of cultural and natural heritage of the Republic of Srpska that works from 1976. There are no confident data on the period of foundation of this object. However, many circumstances point to a conclusion that exactly on this location there was a Roman settlement Castra. The Romans were exposed to frequent rushes of barbarian nations, and they had strong reasons to defend the road that passed through the basin of the river Vrbas. On behalf of such statements there are archaeological findings that were discovered on the territory of current Kastel, and it refers to the Roman ceramics, money and architecture. A particularly important finding is the antique altar dedicated to the God Jupiter, found in 1885 during the construction of the bridge across the river Crkvena. Further on, on the territory of the fortress remnants of the Slavic settlements from the period of early Middle Age were found (from VIII to XII century). In the Bosnian medieval state there was a Vrbas city, which, according to an assumption, was located on the position of current Kastel. Still, according to the other assumption, the Vrbas city is identified with Podgradci in Potkozarje. Intensive construction of Kastel begins in the last but one decade of the 15th century, in the period of Turkish occupation and the reign of the dynasty of Ferhad pasha Sokolović(1574-1588) that apart from this fortification built series of other objects of oriental type. Ferhad-pasha firstly built his fortified tophane (arsenal) on the place of current Kastel, in order to, in some ten years later, turn the tophane into real fortified city with towers and tabiyas, which was constantly additionally built. Since it lies on the mouth of Crkvena into Vrbas, two bridges were constructed from the fortress. One lied across the river Vrbas, near the current city bridge, and the other across the river Crkvena. The bridge across the river Vrbas is saved only in one of the old engravings. The rivers Vrbas and Crkvena were connected by one huge retrenchment (ditch) so that the fortress was actually, at the time, a fortified isle surrounded by the waters of the two rivers. On the walls of the fortress there was also a large wooden billet that looked out to the river Vrbas, as well as the series of built objects from the period when the Austro-Hungarian army was in the fortress. The fortress, as well as the City of Banja Luka, especially gained on the significance in the period of the Austro-Hungarian-Turkish wars as an important geostrategic center. In the plans for recovery, restoration and revitalisation of Kastel it was anticipated that one part of these objects be restored, so that the fortress could have the appearance as similar as it was possible to the old one. At the area of the fortress itself, a great variety of archaeological traces were found, from antique period, over the mentioned Slavic settlement, to newer cultures. The Roman sarcophagus, the Roman milestone from the old road Salona – Servitium, which was found in Šargovac, can be found on Kastel. Today Kastel is a monument of the first class and a place where you can experience the spirit of the past time.
In In the area of Banja Luka and its surroundings, one can follow a continuous development of human settlements from the prehistoric period until the present time. The area was first settled thanks to its rich natural resources and later due to its suitable geographic, traffic and strategic position. The name of Banja Luka was first mentioned in the chart signed by the Hungarian King Ladislaus II Jagiellon in 1494, written in Latin and issued in Buda (present Budapest), although the city had existed even earlier. Prehistoric archaeological sites and the objects found prove that there were human settlements in this area in the period of Mousterian back to 50.000 – 35.000 B.C. In the antique period, the wider area of Banja Luka and western Bosnia was inhabited by the ancient tribes of Illyrians, known as Maezaei and Oseriates, that left numerous forts in the area. During The Great Illyrian Revolt (6-9 A.C) the Romans conquered the Illyrians and founded the Illyricum province. A part of their administrative and military structure was the development of the network of roads along which many military camps (castra) and civilian settlements (municipia) were established.
After the fall of Roman Empire the area was settled by the Slavs who left their early Slavic forts. In the medieval period Banja Luka and its surroundings flourished again, the respective findings can be seen in many written documents as well in the remnants of many fortified cities from XII to XV century. After the fall of medieval Bosnian state and the arrival of the Turks in this area in 1525, Banja Luka gained the importance as the strategic stronghold under the interests of both Turkish and Hungarian Empire. Banja Luka became particularly important during the reign of Ferhad-pasha Sokolović (1574-1588) when it became the seat of the Turkish administration unit (Bosnian Pashaluk). After 350 years of the Turkish occupation the town became a part of the Austro-Hungarian Empire that ruled in this area for 40 years. After the World War I, the area became a part of the Kingdom of Serbs, Croats and Slovenians, and In 1929 Banja Luka became the capital of the Vrbas Banovina of the Kingdom of Yugoslavia, when the town reached its apogee. The first ban of the Vrbas Banovina, Svetislav Tisa Milosavljević (1929-1934) built many buildings in the city during his reign. The most significant are: Administration building and Banski dvor, the National Theatre, Palace Hotel, Sokolski Dom, City Park, Ethnographic Museum, schools, hospitals, etc. After the World WarII, Banja Luka flourished again but its development was stopped during the great earthquake in 1969 and the war 1992-1995. Today, Banja Luka is the second largest city in Bosnia and Herzegovina and a centre of the economy, education, administration and politics in the Republic of Srpska (Bosnia and Herzegovina entity).