A vague item from news.bg seems to be describing a cistern system:

Ancient depot and water depository were revealed by archaeologists in the ancient Thracian city of Perperikon (Rhodopes).

This was reported by the chief of the expedition professor Nikolai Ovcharov. The water depository is situated on the very same top of Perperikon.

The reservoir is 12 m long, per 6 m wide and deep; it used to gather round 270 c m water.

Professor Ovcharov and his colleagues found also 4 completely preserved jars with 160-200 l capacity. Probably the jars had been used for preserving of wine, believes the archaeologist.

... the coverage from Novinite is a bit better:

Bulgarian archaeologists announced they have discovered the biggest ancient tank for storing water on the Balkans, etched into the rock sanctuary of Perperikon, near Kardzhali in southern Bulgaria.

Top archaeologist Nikolay Ovcharov, who unearthed the water tank to add it to the long list of exciting finds from the rock sanctuary, says the discovery proves that there were times when Perperikon was densely populated and with huge water supplies.

The tank, measuring twelve-meter-long, six-meter-wide and six-meter-deep, has a capacity of 432 000 litres.

It was just last month that Ovcharov showed the press two unique ceramic figurines of a cobra, dragon heads and a throne with an upright phallus that were discovered at the rock sanctuary of Perperikon.

The city of Perperikon has been inhabited since around 5000 BC, while a nearby shrine dedicated to Orpheus, near the village of Tatul, dates back to 6000 BC and is older than the Pyramids of Giza.