The Kastel Fortress in Banja Luka 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.
The site has been fortified since Roman times, but the current walls and architecture were made mainly in 1490s by Ottoman reign.
The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.