Worlds of Magic: Planar Conquest (JP)

A very straightforward trophy but one that does take awhile to achieve. The faction that you choose for your game has positive and negative effects. One of these is how fast a city population can grow. If you are going specifically for this trophy then choose a faction that has positive city growth.

Once you get towards the end of your game, eventually get as many cities as you can and then put as many of them as you can on Housing for building as well as putting all citizens into Workers. This simply makes the trophy progress go faster.

If you are fortunate enough to research spells such as Divine Fertility then you can increase population growth even more.

You can check the status of all your towns in the Cities Management menu. I did confirm that the trophy immediately unlocks once the total of all your cities is over 100,000.

Note: The total you would have to add yourself as the game only displays individuals totals not empire totals.