Albert Barnes Commentary Numbers 21:15

"And the slope of the valleys That inclineth toward the dwelling of Ar, And leaneth upon the border of Moab." — Numbers 21:15 (ASV)

To the dwelling of Ar - Ar (Isaiah 15:1) was on the bank of the Arnon, lower down the stream than where the Israelites crossed. Near the spot where the upper Arnon receives the tributary Nahaliel (Numbers 21:19), there rises, in the midst of the meadow-land between the two torrents, a hill covered with the ruins of the ancient city (Joshua 13:9, Joshua 13:16).