2 Corinthians 10:4 - The weapons we fight with are not the weapons of the world. On the contrary, they have divine power to tear down strongholds.
Wherever a stronghold exists, it is a deomonically induced pattern of thinking. Specifically it is a "House made of thoughts" which has become a dwelling place for satanic activity. The strongholds upon which demons relay consists of our own thoughts, attitudes, and opinions which are in agreement with evil.
The strongholds that affect us most are those which are so hidden in our thinking patterns that we do not recognize them or identify them as evil.