"For though we walk in the flesh we do not war after the flesh for the weapons of our warfare are not carnal but mighty through God to the pulling down of strongholds, casting down imaginations and every high thing that exalts itself against the knowledge of God and bringing into captivity every thought to the obedience of Christ" ---(2 corinthians 10:3-5).

From the above bible verse mentioned three vital things, they are "stronghold, imaginations, and thoughts". When a thought is unchecked will authomaticaly become an imagination. And when we say "imagination" we can break down the word and say "image" A thought is going to produce an image. Just like saying a word "cat", surely you don't see the letter c-a-t, rather you see your Chihuahua or whatever kind of cat you have. That is what you see. you see a picture.
Likewise when you hear a devilish thought "sickness" you see a picture maybe you see a funeral. Maybe you see a hospital bed. maybe you see yourself dying and falling apart. When the devil speaks a thought into your mind it creates a picture. When that thought is meditated on, over time it creates a vivid picture, and that imagination becomes a stronghold.

What is a stronghold?
A stronghold is something that has a "strong" "hold" over you. So it starts as a thought, then it grows into an image, and that's when it becomes a stronghold.
So if we want to tear dwon strongholds, we've got to deal with our imagination or our image. And if we want to deal with the images we have to deal with our thought life.

People can portray a godly life on the outside, we have to deal with our thought life on the inside, because that's who we really are. That's what really going to manifest in our lives and that is why our thoughts must be on what God thinks about us. We have got to develop the right image or imagination on the inside. We must understand the image of who we really are onthe inside and then meditate on that image. That image will give us a "stronghold" over whatever the devil is trying to do in our life, rather than it having a strong hold over us.