Tvrdja Osijek complex was built in the 18th century with impressive defensive walls and town gates, Romanic nucleus and rectangular main square, so the Osijek is the most important military, economic, administrative and cultural center of Slavonia. Tvrdja belongs to most important old Croatian town nucleuses and it has the greatest harmony of Baroque military buildings preserved until today. Standing out is the main guard building from 1730, with arcades and a square tower for guard patrols.
Elongated one-story barracks are known under names like Neutor, Donati, Zeichenamt, Jugend Kaserne. Preserved in Tvrdja are beautiful aristocratic and citizen buildings, Baroque circular pillar of the Holy Trinity (1729-1730), fountain, Museum of Slavonia building, old gymnasium, church of St. Michael with valuable inventory and accessories (kept here is the chalice with enamel medallions with images from the life of St. Francis Xavier, with a stamp from Vienna from 1764).