1 Samuel 23:7

And it was told Saul, who had his spies watching all the movements of David, that David was come to Keilah. And Saul said, God hath delivered him into mine hand, for he tried to deceive himself into believing that David, and not himself, had been rejected by God; for he is shut in, by entering into a town that hath gates and bars. Saul thought that David had prepared a trap for himself by making a fortified city his headquarters, since escape would there be more difficult, once the city was surrounded.