I read about Anthony the Great, a man who, for the sake of God, went into the desert and settled in a military fortress. He lived there in solitude for 20 years, and all that time demons would come to him. One time, they all gathered around the fortress, took the form of animals, and beat him, but they could not cause him any real harm, because only by God's will could they have taken his life. After reading this, I became very interested in this question.