The Museum of Slavonia occupies the 18th-century Magistrate building on the main square of Tvrđa, and is the largest general museum in Croatia. The collection covers the archaeology, natural history, ethnography, and cultural history of Slavonia from prehistoric times through the modern period.
The archaeological section is strong — Roman artefacts from the legionary fortress of Mursa (which preceded Osijek on the same site) are among the highlights. The ethnographic rooms reconstruct traditional Slavonian domestic interiors and display folk costumes, textiles, and pottery. The building itself — a long baroque facade on the main square — is worth seeing even if you don't go in. Open Tuesday to Sunday.