Spiritual strongholds begin with a thought. One thought becomes a consideration. A consideration develops into an attitude, which leads then to action. Action repeated becomes a habit, and a habit establishes a "power base for the enemy," that is, a stronghold.
When I write a book, I put everything I have into it; so the more I have, the more the books become. Some people get freaked out by them: mostly the people who believe, mistakenly, that fantasy is about escaping reality. To them I say: If you have a problem with reality, you should be spending more time dealing with your life, and less time reading popcorn fantasy.