Albert Barnes Commentary 2 Kings 18:8

"He smote the Philistines unto Gaza and the borders thereof, from the tower of the watchmen to the fortified city." — 2 Kings 18:8 (ASV)

Sargon had established the complete dominion of Assyria over the Philistines. Therefore, the purpose of Hezekiah’s Philistine campaign was not so much conquest as opposition to Assyrian power. The success of this campaign is indicated in the Assyrian records by the number of towns in this region that Sennacherib recovered before he advanced on Jerusalem.