Bosphorus

The Bosporus winds between Europe and Asia, linking the Black Sea with the Sea of Marmara. Beyond that inner sea, the Dardanelles opens the route toward the Aegean and Mediterranean, so the strait joins an unusually long chain of waterways between the Black Sea and the wider ocean. Istanbul spreads along both banks, but the navigable passage itself is constricted: its minimum width is about 700 metres, making it the narrowest strait used for international navigation.

Control of this passage mattered greatly to Constantinople. On the Asian shore, the Ottomans built Anadolu Hisarı under Bayezid I; in 1452 Mehmed II added Rumeli Hisarı on the opposite bank and strengthened the older fortress. The paired works allowed the Ottomans to control ships passing through the strait before the siege of Constantinople in 1453.

The Golden Horn, an inlet off the strait, offered Constantinople a sheltered anchorage. Together with the passage between the seas, it made the city a meeting point for ships, supplies and political power across the eastern Mediterranean and Black Sea worlds.