John Calvin Commentary Acts 28:16

"And when we entered into Rome, Paul was suffered to abide by himself with the soldier that guarded him." — Acts 28:16 (ASV)

The centurion delivered the prisoners. Luke signifies that Paul had more liberty granted to him than the rest, for his condition and situation were unique. For he was allowed to live in a house by himself with one guard, while the rest were confined in the common prison.

For the general captain knew from Festus’ report that Paul was guiltless, and the centurion, as is likely, faithfully recounted such things as might serve to bring him into favor.

Nevertheless, let us recognize that God governed from heaven the bonds of His servant, not only to ease him of his trouble, but also so that the faithful might have freer access to him. For He did not want the treasure of his faith confined in prison, but He wanted it made accessible, so that it might enrich many far and wide.

And yet Paul was not so free that he did not always carry a chain. Luke calls the general captain στρατοπεδαρχης, who was appointed over the army that guarded the city, as histories mention.