I have read many times that the only way for Europeans to get to China, or vice versa, was through Anatolia and Persia. This gave Istanbul an advantage in throttling trade, as well as Mesopotamia and locations in Persia.
I am sure that due to climate considerations, those were the easiest routes, but when those were blocked by unfriendly regimes, why didn't trade or travel go north of the Caspian and Black seas? Through what today is called Kazakhstan and Ukrain? Is there a reason that it is particularly difficult to go that way? Or am I mistaken? I have never read about any significant traffic that way. To the contrary, usually it is stated that the only way to China was through Persia.