Kastel fortress is located in Banja Luka and it is one of the oldest buildings in Bosnia and Herzegovina. There is little information about who and when it was built, but it is believed to have been constructed by the Romans during the 3rd century; however, some finding argue that the fortress was built much later.
The structure is surrounded by stone walls with loopholes and observation posts which shows that it was a quite strong military fortress.
The excavation of various items and coins near the fortress reinforce the theory that it was the Romans who used the fortress.
In 2013, numerous items were found from the 17th and 18th centuries, including a very well-preserved egg.