Paul gives his credentials apart from Christ. He recounts his conversion and call and makes claims that some of them would recognize.
His true defense is his testimony. God chose him to know his will, to see his righteousness and to hear his voice. He would be a witness of all he's seen and heard.
Paul recounts a vision in Jerusalem that amounted to his commissioning. We'd heard his conversion, but not this vision. This is news to us. He argues, apparently, that he should stay in Jerusalem because everyone would see the change -- God says go. Twice he speaks of being appointed or chosen.
He is shouted down and beaten by authorities then he appeals to his Roman citizenship which strikes fear in the heart of his captors.