The Mystery of the Double-Headed Coins of Ancient Istros Dobrudja coast of the Black Sea. Situated approximately 300 miles north of Byzantium, this small city was founded by Miletian traders around 657 or 656 BCE. As the oldest Greek colony on the Black Sea, Istros survived for some 14 centuries before its eventual abandonment in the mid-seventh century CE. It is unsurprising, therefore, that such a long-lasting settlement would produce several interesting and important numismatic types. Known as the “initiator and main producer” of the distinctive cast bronze arrowhead proto-money in the seventh and sixth centuries BCE, the Istrians used this currency to slowly built their city into a “busy commercial center”.