John Gill Commentary Judges 1:34

"And the Amorites forced the children of Dan into the hill-country; for they would not suffer them to come down to the valley;" — Judges 1:34 (ASV)

And the Amorites forced the children of Dan into the
mountain
Into the mountainous part of the tribe of Dan; the most noted mountains in it were Sear and Baalah, which lay on the border of Judah, (Joshua 15:10Joshua 15:11); Joppa in this tribe was built on an high rock, and so Gibbethon, as its name seems to import, perhaps was built on a hill or mountain, as were the cities after mentioned:

for they would not suffer them to come down to the valley ;
which lay between Joppa and Caesarea, the plain of Sharon, in which were Lydda, Jamnia which belonged to their tribe, and they afterwards enjoyed; (See Gill on Joshua 19:48).