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 all the way back to Roman and even pre-Roman times. 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.