Built by the Austrians in 1882 on the site of a 17th-century fortress above a 70m ravine on the Koritnica River, Kluže Fortress is 4km northeast of Bovec. It was the site of an Austro-Hungarian garrison during WWI, right behind the front line of the Isonzo battlefield. Exhibitions outline its turbulent history. Even more dramatic is the upper fortress, Fort Hermann, built in 1900 halfway up Mt Rombon to the west.
You can walk to these ruins from Kluže in about half an hour.