Albert Barnes Commentary Judges 18:28

"And there was no deliverer, because it was far from Sidon, and they had no dealings with any man; and it was in the valley that lieth by Beth-rehob. And they built the city, and dwelt therein." — Judges 18:28 (ASV)

Rehob (as Dan was later) is mentioned as the northernmost point of the land of Canaan (Numbers 13:21), and its position is defined with reference to the entrance of Hamath.

A city – Rather, “the” city. They rebuilt Laish, which they had burned down (Judges 18:29).