Ruins of fortress Krivus are in the South of the Rhodope mountains, in the district of Kardzhali, near the village of Bachevo. They are located on a rocky promontory a hundred metres above the river Arda, surrounding three sides of the ancient fortress.

Historians believe that the medieval Bulgarian fortress Krivus was built in the 10th century. The location was not chosen by chance – the fortress was inaccessible due to the surrounding steep cliffs. The main function of the fortress Krivus, which is located near the castle of Patmos was the defense of the area around the Arda river in the Eastern part of the Rhodopes. Finally destroyed the citadel was, like most Bulgarian fortifications, during the invasion Osman. History has kept mentioning that during the Turkish slavery, the fortress was used as a women's prison.

To survived and are in good condition the fortified wall, which has a thickness of approximately 2.5 metres at the base and 1.75 at the top and a height of about five meters, defensive towers and two well-fortified entrance to the fortress. Also well preserved citadel - inner fortress – sizes 40 to 50 meters and its surrounding jagged wall, six feet in height. In the center of a small courtyard, the Church was built, but it remained only ruins. Between the river and the underground fortress was built a special tunnel, whereby the territory of the fortified defenses were filled with water. It was created in case of siege, enemies and was a unique example of engineering of the era. Today the tunnel is buried and difficult to find.

The excavations of the ancient fortress of Krivus I found lots of old coins, jewelry, and Christian symbols, such as stone crosses, and weapons. The fortress is repeatedly mentioned in Byzantine Chronicles George Acropolita. Today the ruins are open to the public. From the top of the hill offers a picturesque view.