Ključ

Ključ - Credit: Wikimedia Commons

Ključ Fortress is a medieval fortress in Bosnia and Herzegovina, above the modern-day town of Ključ. It is located on an elongated slope on high rocks that dominate the valley of the river Sana. Before it was redeveloped, there was a small ancient fortress from Roman times. Read more on Wikipedia

type: military installation
date from: Unknown-12th century

Source: en.wikipedia.org