Kastel Fortress, Medieval fortress in Banja Luka, Bosnia and Herzegovina.

Kastel Fortress is a stone fortress in Banja Luka built with thick defensive walls, watchtowers, and artillery positions positioned along the Vrbas River. The complex includes an underground passage system, a restaurant, and various defensive structures that visitors can walk through.

The site was a Roman military camp called Castra, most likely built in the 2nd century AD on the salt road between Salona and the Sava. After Slavic settlement the fort was destroyed, and in the Ottoman period Kastel took its present form, with final dimensions set in 1714 under Numan-pasha Ćuprilić. In 1737 Austrian troops under General Hildenburgshausen besieged it but were routed by Ali-pasha Hekimoglu.

The fortress's name appears in a 1525 charter as "Castel Nostro Banja Luka". Its walls now host Motofest Banja Luka, the region's largest motorcycle fair, held annually inside the grounds. The Republic of Srpska's Institute for the Protection of Cultural, Historical and Natural Heritage has occupied rooms here since 1976.

The grounds are easy to explore on foot and the underground passages are accessible during regular hours. There is parking available on-site, and the restaurant inside offers a good place to rest and refuel.

Excavations of the ramparts in 2013 uncovered objects from the 17th and 18th centuries, among them a completely preserved egg. The site also yielded flint tools from the Gravettian period of the Upper Paleolithic and Neolithic pottery fired to a black and grey finish.