We are told that the Seven Heads of the Beast represent Seven Mountains; then we are told right after that, that the Seven Heads also represent Seven Kings of whom "Five are fallen and one is and the other is not yet come; and when he cometh he must continue a short space."

             That is, in John's day "Five of these Kings had fallen, one was the ruling Emperor and the Seventh was yet to come.

                        Who the fallen five kings are, are not known.  The one who was ruling was the one in John's day and was the one who had banned John to the Isle of Patmos; and the one who is yet to come has to be the Antichrist.