Jamieson, Fausset and Brown on Revelation 16:19

19. the great city--the capital and seat of the apostate Church, spiritual Babylon (of which Rome is the representative, if one literal city be meant). The city in Re 11:8 (see on Re 11:8), is probably distinct, namely, Jerusalem under Antichrist (the beast, who is distinct from the harlot or apostate Church). In Re 11:13 only a tenth of Jerusalem falls whereas here the city (Babylon) "became (Greek) into three parts" by the earthquake.

cities of the nations--other great cities in league with spiritual Babylon.

great ... came in remembrance--Greek, "Babylon the great was remembered" (Re 18:5). It is now that the last call to escape from Babylon is given to God's people in her (Re 18:4).

fierceness--the boiling over outburst of His wrath (Greek, "thumou orgees"), compare Note, see on Re 14:10.