The ancient Thracian city of Perperikon (also Perpericum; Bulgarian: Перперикон, Greek: Περπερικόν) is located in the Eastern Rhodopes, 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 (“Upper Fortress”) 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.

+Info : https://en.wikipedia.org/wiki/Perperikon