“And the hand of the Midianites was strong against Israel; from before the face of Midian, the Sons of Israel made for themselves the caves which are in the mountains — the caves and the fortified places.”
The presence of the definite article before “caves” is significant. It implies that these are not just random, natural caves that were found in the mountains, but a network of caves for soldiers to hide in (reference to them being fortified) of which later readers would have been aware. Similarly, the word choice here emphasizes that these caves were either created by the Israelites or perhaps (as some scholars suggest), these were natural caves that were enlarged and adapted to the soldier’s needs. In any sense, the soldiers were creating a place of refuge where they could hide until God raised up a Judge to lead them into battle.
An interesting point of note is that the term we translate as “fortified places” is dDxVm (metsad), which is the root word behind the term Masada, the fortress destroyed by the Romans in 73 AD due to the resistance of the Zealots and the Sicarii. Like these fortified places in the book of Judges, Masada was a nearly impregnable fortress at the top of a great hill where soldiers dug in waiting for the Messiah to lead them into glorious battle against their Roman rulers. Things change very little over the years.
While I do not mean to undermine the military importance of these strongholds, it raises the question: when the soldiers flee to the mountains, what happens to the people of the land? Well, the answer can be found in the verses that follow — they were overrun and the Midianites consumed all of their resources, leaving them to starve. Had people been obedient in the first place, how much suffering could have been avoided. Were the soldiers to have remained amongst the people, to lead and protect them from the marauding Midianites, how much better off people would be. When the strong go into hiding, the people suffer. When men of faith get busy with the things of this world, the church suffers as well.