Albert Barnes Commentary Judges 20:42

"Therefore they turned their backs before the men of Israel unto the way of the wilderness; but the battle followed hard after them; and they that came out of the cities destroyed them in the midst thereof." — Judges 20:42 (ASV)

The way of the wilderness - that is, the wilderness which extended from Jericho to the hills of Bethel.

Those who came out of the cities - These must be the Benjamites (Judges 20:15). Hence, “in the midst of them” must mean “in their own cities,” where they each fled for refuge but failed to find shelter (Judges 20:48). Anathoth, Alemath, Ramah, Ataroth, Geba, Michmash, Ai, Bethel, Migron, etc., would probably be the cities meant, all lying east and north of Gibeah.