William Burkitt on Acts 12:4

Peter being imprisoned, observe, What an excess of care and caution Herod takes for securing his prisoner: Sixteen soldiers are set to guard him; four at a time, and to take their turns at the four watches of the night, to relieve one another.

Thence learn, That the enemies of the church make the surest work they can, when at any time God permits his servants to fall into any of their hands. Thus they dealt with the Head, when they had apprehended him; The watch-word was Hold him fast, Matt 26:48. When they crucified him, they nailed him fast to his cross: when they buried him, they made the sepulchre sure, sealing the stone, and setting a watch: And thus they deal with some soldiers chained to him, and others always standing at the door to observe him. But all this care, concern, and caution to secure the prisoner, did illustrate the glory of the miracle in his wonderful deliverance.