To pay his riddle wager, Samson killed thirty men from which city?

Answer: Ashkelon

After his wife revealed the answer, Samson went to Ashkelon, killed thirty men, and gave their clothes to those who solved the riddle.

“Then the Spirit of the LORD came mightily upon him, and he went down to Ashkelon, killed thirty of their men, took their apparel, and gave their clothes to those who had solved the riddle. And burning with anger, Samson returned to his father’s house,”— Judges 14:19 (BSB)