The Appony Castle ruins stand in the hills above Oponice, north of Nitra. The medieval fortress belonged to the Apponyi family, was reshaped over several centuries, and has been a ruin since the early modern period.
This is a light hike and history detour rather than a staffed attraction. Go for the forest approach, broken walls, and the way the site connects Nitra's lowland towns with the old defensive line of the Tribeč hills.