John Gill Commentary Acts 16:24

"who, having received such a charge, cast them into the inner prison, and made their feet fast in the stocks." — Acts 16:24 (ASV)

Who having received such a charge So strict an one from the magistrates, was doubly careful and diligent:

thrust them into the inner prison : the innermost part of it, the lowest or furthest part of the prison; so that there were the more doors, bolts, and bars, to break open, and pass through, should they attempt to make their escape:

and made their feet fast in the stocks ; or "wood", a wooden machine, in which the feet of prisoners were put for security, and which we call "the stocks"; some say it was such an one, in which the neck as well as the feet were put, and so might be the same with our "pillory".