The concept of a stronghold is foreign to our modern minds, except for what we may have read about it in history books. It is not part of our frame of reference and because of this, we may not understand the significance of this. We need to understand the physical structure in order to understand what was meant when it was employed as a metaphor.
Each of us has a stronghold in our lives. When we have the right kind, it is to our benefit, but some strongholds are to our detriment.