A "stronghold" is a mind-set that holds you hostage. It makes you believe that you are hopelessly locked in a situation, that you are powerless to change.
That’s when you hear people saying, “I can’t, I can’t, I can’t.” The only reason you say, “I can’t,” when God says, “You can” is that Satan has made himself at home in your head and you are allowing him to stay rent-free!
In computer terms, he has you operating by the old information that was on the hard drive of your mind before you became a Christian!