Ostrovica Bosnia & Hercegovina

Crowning the forested mount behind Kulen Vakuf, this medieval fortress is much more ruinous than it might appear from below. The interior is almost impossibly overgrown with nettles and brambles, but the views down over the village and Una Valley are hard to beat. Climbing on foot takes around an hour, or you can drive up a convoluted 5.5km unpaved road; the last 500m gets pretty narrow. The castle was first mentioned in 1407 as the property of a Bosnian duke. It fell to the Ottomans in 1523 and reached its zenith as a regional command centre in the early 18th century.