​What is a "stronghold" ?

A "stronghold" is simply a word picture describing the believer "caught" in a particular sin with seemingly no way of escape...the sin has a "strong hold" over them (it is spiritual bondage). Another way to understand this concept is when a particular sin or failure becomes a habit, grows to a compulsion that can end up as a full blown addiction. Often it's where your flesh tendencies intersect with satan's lies (1 Peter 5:8) that you believe or accept over time resulting in a powerful and sometimes seemingly irresistible pull toward sin. The foundation of just about every stronghold is "the lie". What happens? You believe and accept a lie about yourself, life, God or others and then, yield to some fleshly expression of that lie; absent the intervention of "truth", you succumb to the fleshly expression of that lie and develop/receive compounding lies that further strengthens the bondage ("it's just the way I am", "I can't help it", "it's no big deal", "it helps me cope" or other compounding lies). At this point, satan and his forces are masterfully controlling and influencing your life in this area (No, you're not possessed as a believer but you may sure act like it). Interestingly, as believers we often focus on the behavior and try to "fix" that stronghold (and there may be some societal value in that) yet the real strong hold to address is the lie(s). I would agree that often we need to stop the bleeding on the outside so we can have presence of mind to deal with the inside lies, but don't spend too much time outside. Attack the stronghold at it's foundation (the lie), resist the enemy by the authority you have in Christ (James 4:7), proclaiming the truth and trusting Christ for the victory in your behavior (Colossians 2:6).