Jamieson, Fausset and Brown on Zechariah 9:3

3. The heathen historian, Diodorus Siculus [17.40], confirms this. "Tyre had the greatest confidence owing to her insular position and fortifications, and the abundant stores she had prepared." New Tyre was on an island seven hundred paces from the shore. As Isaiah's and Ezekiel's (Eze 27:1-36) prophecies were directed against Old Tyre on the mainland and were fulfilled by Nebuchadnezzar, so Zechariah's are against New Tyre, which was made seemingly impregnable by a double wall one hundred fifty feet high, as well as the sea on all sides.