How do I create multiple save points of a fortress?
The game intentionally does not allow you to quit without saving (or abandoning) and doesn't allow you to have more than one save per fort. The idea is that there is no going back. Losing is supposed to be fun.
However, you can get around this problem with a little trickery. Save the game. Then make a copy of your game folder. Then keep playing. If you want to load the previous save, just delete your current game folder and replace it with the old one. It's a lot of work, but it gets the job done.
You can "save-scum" like this. Look in the folder (dwarf fortress folder)\data\save. In it you'll find at least two folders. One is called "current" and should always be empty. Each world you are playing will also have a folder, called region1, region 2 and so on. Each of these folders stores the complete state of that world. Duplicate any of these folders to create a duplicate copy of the world.
You can also turn on autosave. In the file (dwarf fortress folder)\data\init\d_init.txt the autosave options are near the top. Change them to
and every season the game will briefly pause as it saves the current state of the world. These saves will end up along with the other world folders as folders with names such as region1-aut-126, region1-aut-127, region1-spr-126 and so on, referring to the season and they were saved.
When I want to go back to one of these autosaves, I rename or delete the region1 folder and then rename the folder I want so that it becomes the new region1. Otherwise things get confusing because the backups start getting names like region1-spr-126-aut-128 and it stops being obvious which is the up-to-date save.
If you're playing an older version of dwarf fortress, you may not have a d_init.txt file. If so, look in init.txt instead.