The Bosphorus is the strait that separates Europe from Asia, and a trip along its shores is one of the essential Istanbul experiences. Both sides of the channel are lined with old Ottoman wooden villas, 19th-century palaces, and waterside restaurants where you can watch tankers and ferries glide past.
The cheapest way to see it is on a regular commuter ferry — for a few lira you get the same views as the expensive tourist boats. From the water you will spot the twin fortresses built by Sultan Mehmet the Conqueror in the mid-15th century to control the strait before his siege of Constantinople: Rumeli Hisari on the European side and Anadolu Hisari on the Asian side.
If you want to walk the shores, the Ortakoy neighbourhood is a good starting point — a mosque on the waterfront, cafes and bars around the square, and views across to Asia. The Bosphorus at night, lit up with the bridges and palace lights reflecting on the water, is unforgettable.