Read this article and answer this question The Azov campaigns of 1695-96 , were two Russian military campaigns during the Russo-Turkish War of 1686-1700, led by Peter the Great and aimed at capturing the Turkish fortress of Azov , which had been blocking Russia's access to the Azov Sea and the Black Sea.  Since the Crimean campaigns of 1687 and 1689 had failed because of the difficulty of moving a large army across the steppe, Peter decided to try a river approach.
How many seas did the Turkish fortress block off?
2