This term designated a high-ranking military officer, specifically the camp commander or prefect in Roman military organization. The praefectus castrorum ranked among the senior officers of a legion, responsible for camp administration, logistics, and sometimes prisoner custody. When Paul arrives in Rome, he is handed over to this commander of the guard. The textual variant here affects which ancient manuscripts include this detail, but the term clearly denotes a senior military authority responsible for supervising important prisoners. The position's prestige indicates Paul's treatment as a significant, not merely common, prisoner.