John Gill Commentary 1 Samuel 23:7

"And it was told Saul that David was come to Keilah. And Saul said, God hath delivered him into my hand; for he is shut in, by entering into a town that hath gates and bars." — 1 Samuel 23:7 (ASV)

And it was told Saul that David was come to Keilah
No doubt it was told him what he came thither for, to relieve it, and deliver it out of the hands of the Philistines, and what success he had; which one would have thought would have reconciled his mind to him, and made him think well of them; but instead of that, it only led him to contrive mischief against him:

and Saul said, God has delivered him into my hand ;
as if the success he had given to David was against him, and in favour of Saul:

for he is shut in, by entering into a town that has gates and bars ;
while he betook himself to caves, and fields, and woods, he had no great hopes of finding him out, and coming up with him, and seizing him; but now he had got into a fortified place, enclosed with walls, and that had gates to it, kept bolted and barred; when he brought his army against it, and surrounded it, he imagined he would not be able to get out, and escape his hands.