The Kastel Fortress (Serbian Cyrillic: Тврђава Кастел, romanized: Tvrđava Kastel) is a fortress located in Banja Luka, Bosnia and Herzegovina. The fortress is medieval but is situated on the site of previous fortifications going back to Roman and even pre-Roman times. It was captured by the Ottoman Empire in 1527. The fortress is relatively well-preserved, and is one of Banja Luka's main attractions, situated on the left bank of the Vrbas river in the very center of town.