This museum occupies a former granary and part of the fortification walls of the medieval town, dating from the 5th century AD. At this time, Sozopol was considered an important defensive post of the Byzantine Empire, with its capital at Constantinople (Istanbul). Visitors pass by some interior defensive walls and a well going back to the 3rd century BC. That said, it’s a bit underwhelming. Even if you skip the museum, walk under the arch to see the sea views.