Albert Barnes Commentary Judges 6:2

"And the hand of Midian prevailed against Israel; and because of Midian the children of Israel made them the dens which are in the mountains, and the caves, and the strongholds." — Judges 6:2 (ASV)

The word rendered “dens” is only found in this passage. It is best explained as ravines hollowed out by torrents, which the Israelites made into hiding-places.