The word rendered “dens” is only found in this passage. It is best explained of ravines hollowed out by torrents, which the Israelites made into hiding-places.
Related Commentaries of Judges 6:2
Judges 6:2
2 And the hand of Midian prevaileda against Israel: and because of the Midianites the children of Israel made them the dens which are in the mountains, and caves, and strong holds.