Selymbria (modern Silivri, Turkey) is city in Thrace on the north shore of the Sea of Marmara, west of Constantinople, inside the Long Walls of Thrace. Procopius attributes the fortification of Selymbria to Justinian I, and Theophanes also states that Justinian went to Selymbria "to build the Long Wall." Selymbria was an important strategic point at the end of the Via Egnatia and is usually mentioned in connection with the passage of armies and processions: the dying Constantine V was brought from Arkadioupolis to Selymbria, where he boarded a ship, and Nicholas I Mystikos invited Symeon of Bulgaria to come to Herakleia or Selymbria to negotiate peace. Manuel I spent Easter of 1167 at Selymbria on his way to Hungary.
The city acquired special significance during the civil wars of the mid-14th century. John VI Kantakouzenos rebuilt its fortifications, and the remains of his ramparts still stand. In 1345 the wedding of John V's daughter to the Ottoman sultan Orhan was celebrated in Selymbria. In 1327 Alexios Apokaukos  was archon of Selymbria, and around 1399 a certain Bryennios Leontares acted as kephale of the city. In 1382 John V ceded Selymbria, together with Herakleia, Rhaidestos, and Panion, to Andronikos IV and John VII. In 1453 Selymbria effectively resisted Turkish attack and surrendered only after the fall of Constantinople. 
Selymbria is listed in notitiae as the "archbishopric of Europe," and from the 12th century onward as a metropolis without suffragans. Philotheos, metropolitan of Selymbria in the 14th century, noted several churches there, one of which was sponsored by Apokaukos, the ruins of which still exist.