A “city” (whether walled for defence or not) is a cluster of many people living together and of necessity working together for mutual benefit. The split John describes is not meant to have us think of a city physically dividing into literally three (perhaps equal) parts. “Three” is a full number, denoting a fullness of division. The symbolism characteristic of the book of Revelation means that this great city divides against itself, fragments into unworkable bits. This division is reminiscent of the chaos that overcame Babel (Genesis 11:1–9).
19 The great city was split into three parts, and the cities of the nations fell, and God remembered Babylon the great, to make her drain the cup of the wine of the fury of his wrath.