... and according to the headline, that fortress has been found. Whatever the case, the Wikipedia article whence that image came clarifies things at bit, noting this was the ancient Pyrgos, which became the Roman colony of Dueltum. When one knows that, this salient info begins to make sense:
A text inscribed on a sign from the 2nd century AD sheds light on the name's origins, which, literally translated from Latin, sounds like "Bourgos." The text states that in Roman times many fortresses had been built along the frontier of the Roman colony Deultum. Deultum was established in the last years of the reign of Emperor Vespasian (69-79 AD).
During December 2008, extensive research was carried out on the southeast entrance of the fortress on Foros peninsula near Bourgas, where the massive rectangular tower that had guarded the town gates can be recognised. The outer side of the fortress wall is 2.90m wide; 15m of its length seemed to be well preserved, Drazheva has said.
I'm not sure how old that coat of arms is, but I hae me doots that this is the fortress depicted thereon. Nice find though ...
• Fortress depicted on Bourgas city coat of arms discovered (Sofia Echo)