Nebuchadnezzar sieged Tyre for thirteen years. This is relevant because Nebuchadnezzar was not sieging the island (he did not even have a navy). Consequently, he was sieging Tyre's coastal district, and it would not have taken thirteen years to capture it if it was not fortified.
It says in Joshua 19:29 -- “to Ramah and to the fortified city of Tyre.” This is relevant because this is a reference to Ushu (Tyre on the coast).1
Ashburbanipal (King of Assyria, 668-626 B.C.) boasted of having destroyed Tyre, and yet did not mention any walls.1 This is relevant because we might have expected him to mention the destruction of walls if there were any.
By way of response, however, few scholars have found this argument from silence persuasive―the consensus is that coastal Tyre was fortified. (See above). It is explainable several ways (for example, by the walls not being as noteworthy as those on the island.)