You may wish to see an introductory page first.
The influence played by Genoa in the frequent dynastic quarrels which characterized the last centuries of the Byzantine Empire is evident in the kind of concessions the emperors made to the Genoese. Not only trading rights, ports of call, monopoly of minerals (such as alum) and entire islands, but even the two fortresses which controlled the passage through the Bosporus.
XIXth century print showing Anadolu Kavagi
Anadolu Kavagi (most likely meaning Anatolian oak) and Rumeli Kavagi on the European side of the Bosporus were two Byzantine fortresses which were peacefully acquired by the Genoese. They were located near the northern mouth of the Bosporus into the Black Sea.
View of the final section of the Bosporus and of the Black Sea from the fortress (the image shows on the European side, and in the far distance the lighthouse of Rumeli Feneri, another Genoese base)
View of the lower fortress and beyond the Bosporus of Rumeli Kavagi
The site is still fortified and the lower part of the hill is controlled by the Turkish army. There is no evidence left of the fortress (Rumeli Kavagi) which once existed on the European side. A chain laid between the two fortresses blocked the passage of hostile ships.
The upper part of the fortress (called Yoros Castle) is open to visitors. The Genoese lost control of the fortresses soon after the 1453 conquest of Constantinople by Sultan Mehmet II. Prior to starting the siege of the city Mehmet II used a carrot and stick approach towards the Genoese to obtain their neutrality in the forthcoming conflict. He built a powerful fortress (Rumeli Hisar) a few miles south of the Genoese fortresses to undermine their control of the Bosporus and at the same time he let them understand that they would not be affected by his conquest of Constantinople.
Relief above the main entrance (internal side); you can see a similar relief in the image used as background for this page
Mehmet II, once he had achieved his main goal, decided to secure his conquest by curtailing the Genoese influence and in a few years the Genoese of the region had to accept willy-nilly the conditions imposed by the Sultan who left them only some trading privileges.