Click on the photo to enlarge. You can spot the Maiden's Tower, right behind the tanker.
You can see why this city was (and is) such a strategic place for commerce and was the capital of empires. The Bosphorus (or Bosporus, both spellings are correct) links the Black Sea (to the left, beyond the photo) to the Sea of Marmara (to the right, beyond the photo) and thus the Mediterranean. And, of course, Asia is on one side of the Bosphorus (across the water in this photo) and Europe on the other side (where I was staying).
Look slightly South and West of Istanbul and note the Dardanelles's important location: between the Sea of Marmara and the Mediterranean. Another strategic place for battles. (As one of our visitors in Istanbul said, Helen of Troy was probably not the main reason for the Trojan War! More on Troy here, courtesy of the latest archeological investigations.)
I've posted maps for Estadounidenses who don't know geography. ;-)
This map has early 20th century names: Istanbul was still called Constantinople.
Here's a reference map of what we call "The Middle East."
Best online collection of maps, by the way, is the Perry-Castañeda Library Map Collection at the University of Texas.