Unlike cities today (sprawling megapolises), the layout and location of cities of old had to include elements of defence in the face of possible attack. Given the nature of warfare at the time, cities built on mountains and islands were easier to defend than cities built on flat country (think of the location of the city of Tyre and of Jerusalem). The reference to “islands” and “mountains” in Revelation 16:20, then, is not a contrast to the reference to “cities” in Revelation 16:19 but a continuation of the theme of cities (= large conglomerations of people) disintegrating due to the seismic shift God generates in people’s thinking.
20 And every island fled away, and no mountains were to be found.