For your own sake I should advise you to be quiet.

I might tell you once and for all that you can't get out.

The door is a stout one, and the windows are exceptionally well barred.

The men to whom I have assigned the duty of looking after you are in their way honest, though a little rough.

Moreover, they are aware that their own safety depends to a very great extent upon your not getting out.
Believe me, if you do not know already, that there is nothing like fear for making a good watch-dog.