Ustra Fortress, Bulgaria

During the Middle Age, the Ustra Fortress was one of the highest strongholds in the Rhodope Mountains. At 1,114 meters altitude, the foundations of the fortress remain a formidable sight. Built during the 10th Century, this stronghold is located on top of a steep rocky mountain. Visitors can nowadays take a look at the three remaining towers and at the impressive 113 metres long outer wall.
Photo credit: mitko_denev (flickr)