Slaves to Armok: God of Blood, Chapter II: Dwarf Fortress

The Manual: Creating Your World

When you start your first game, you'll notice that it goes through a lengthy world generation process. You only have to do this once in a long while, so sit back and enjoy it.
At first, the game produces several fractally generated landscapes, searching for an ideal balance. You'll notice that there are many rejects, sometimes well over one hundred. This is normal -- you'll be playing several fortresses in this world, so it's important that terrain is evenly distributed.
After the basic terrain is prepared, the game will simulate erosion and run rivers and lakes. It's still possible to have a few rejects at this point.
Once the history animation begins, you are indeed watching the world you'll be playing in. The civilizations that are spreading out and fighting over the ages will be the same ones you interact with when you are running a fortress.
When history catches up with the present time, the animated world map becomes a piece of parchment and you are prompted to continue.