Zvornik Fortress — also known as Kula Grad ("Tower City") or Stari Grad ("Old Town") — is a medieval castle perched on Mount Mladevac above the city, commanding views of the Drina valley and the Serbian town of Mali Zvornik directly across the river. The original structure dates from the medieval Bosnian kingdom; the Ottomans captured and substantially expanded it in 1460 and held it for over three centuries.
The fortress is freely accessible and the climb takes about twenty minutes from the road below. The walls and towers are partially ruined but substantial sections remain standing. The panorama from the top — the wide green Drina, the hills on both sides, the industrial outskirts of the modern city below — is worth the effort. It's the one sight in Zvornik that gives you a sense of why this location mattered strategically for so long.