The parties on They Are Billions can last for hours. And the difficulty makes the evolution of the city very complex. We must constantly think the best way to build the defenses, how to position the walls, elite units, …
So we should be able to back up and go back to where we needed to start over where we have failed. But alas it is not possible! You can only save a game once, when you leave it.
You have to go to Windows Explorer and copy paste the backup files to another directory.
To do this :
Documents / My Games / They Are Billions / Saves.
You must have saved and left the game to make it work.
We can thus predict the place of the next wave. It does not work if you build too much defense on the respective side. The infected will prefer to use another path.