There is a full on battle for your mind being waged everyday of your life. Images, suggestions, voices, music, TV, movies, etc. Much of this designed to implant Satan's lies in your mind. If he can get you to believe lies he can hold you hostage.
Strongholds are areas of thinking not based on truth but lies. Instead of protecting us, they actually imprison us. Unless we renew our mind, we risk continuing to believe those lies and making important decisions based on deception.
Truth is your weapon. Saturation of your mind in God's word is the key that unlocks the various prison cells cunningly designed by Satan. Expose yourself as much as possible to God's word. Read it. Hear it taught. Obey it. Then you will be free.
John 8:31-32 (NLT) 31 Jesus said to the people who believed in him, "You are truly my disciples if you keep obeying my teachings. 32 And you will know the truth, and the truth will set you free."