Perperikon

Perperikon, also Perpericum, is an ancient Thracian city located in the Eastern Rhodope Mountains, 15 km northeast of the present-day town of Kardzhali, Bulgaria on a 470 m high rocky hill, which is thought to have been a sacred place. The village of Gorna krepost is located at the foot of the hill and the gold-bearing Perpereshka River flows nearby. Perperikon is the largest megalith ensemble site in the Balkans. In the Middle Ages Perperikon served as a fortress.

Source: Wikipedia