From the Introduction:
With this game, you and two or more of your friends will get together and tell the story of some fantastic characters and their adventures in an alt-Renaissance style. What is alt-Renaissance? It takes the spirit of the Renaissance and uses it to tell stories of what might have been. Your Renaissance might be a world where Leonardo's flying machine actually flies, or Machiavelli makes a pact with Satan, or the Pope becomes the Emperor of the World. Or it may be a world where women gain political equality with men in the 16th century, where the Reformation never happened, and the Industrial Revolution begins 500 years early.
What's Cool About This Game?
Every time you play Principia, you'll ask a question about your world, such as "Is vengeance just?" or "Does the Duke deserve to rule?" or even "Does God exist?" You'll answer these questions when you play, but only after the adventures and struggles of the characters you invent. Then you'll get to see how your answer changes the world.
You'll also get to see historical characters like Michelangelo and Lucrezia Borgia in un-historical situations like fighting zombies, founding a democratic republic, or crossing the Alps in an airship.
If these ideas appeal to you, you’ll probably like Principia.