The mind constitutes a battlefield where Satan and his evil spirits contend against the Truth and hence against the Believer.
Man’s will and spirit are like a citadel which the evil spirits crave to capture.
The open field where the battle is waged for the seizure of the citadel is man’s mind.
2 CORINTHIANS 10.3-5 for walking in the flesh, we do not war according to the flesh, for the weapons of our warfare [are] not fleshly, but powerful to God for bringing down of strongholds, bringing down reasonings, and every high thing lifted up against the knowledge of God, and bringing into captivity every thought to the obedience of the Christ,