Albert Barnes on Acts 28:16

The captain of the guard. The commander of the Pretorian cohort, or guard. The custom was, that those who were sent from the provinces to Rome for trial were delivered to the custody of this guard. The name of the prefect or captain of the guard, at this time, was Burrhas Afranius. Tacit. Ann. 12, 42, 1.

But Paul was suffered, etc. Evidently by the permission of the centurion, whose favour he had so much conciliated on the voyage. See Acts 27:43.

With a soldier that kept him. That is, in the custody of a soldier, to whom he was chained, and who, of course, constantly attended him. See Acts 24:23. Acts 12:6.