The ruined castle of Onzo sits on a promontory above the village, its ring of medieval walls and two partially surviving towers standing out against the terraced hillsides below Monte Peso Grande. The structure is modest but the setting is evocative — the position was clearly chosen to control the Arroscia valley, and the views from the walls down to the river and across to the surrounding ridges explain why successive lords bothered to hold it.
The castle is freely accessible on foot from the village. The walls are not restored and some care is needed around the crumbled sections, but the basic outline of the fortification is easy to read. This is a place for walkers and history enthusiasts rather than a formal tourist attraction.