The most interesting area of study of Revelation 17 and 18 are translating the English interpretations back into the original Greek, and breaking down the meaning behind those words. Some of the English translations were poorly chosen.
John chose his words very carefully when drafting his account, detailing specifics that are not found in the English translations. Studying the Greek which John used gives us a clearer picture of what John was witnessing while in the spirit.
In verse 7 the angel tells John that he will tell him the mystery of the woman and of the beast that carries her. The angel begins to explain a succession of events surrounding the beast, who carries the woman upon his seven heads. The angel begins with the beast with the mortal wound in verse 8, but does not give John much more information about the woman. We find that the woman has an international reach, sitting on many waters: nations, multitudes of peoples and tongues. Then the angel discloses that the woman who John saw was that great (μεγας) city (πολις) which reigns over the kings of the earth, in the last days. The last verse in the chapter, transitions the narrative back to the woman and continues into the following chapter where John discovers that the woman is not only a great city, but she is also a merchant port.
In the Greek, μεγας, mega translates into English as great, huge, enormous by volume, overtly large by physical size, covering vast amounts of territory.
πολις or, polis for city is defined in the Greek as a walled city, mother city, magistrate city, that dominates well defined territory.. So now we know that the woman is a metaphor for a extremely large city.