Here are some Principia resources:

• Actual Play: Venice is Sinking
• Actual Play: Eastern Europe / Ottoman Empire
• Principia posts on Attacks of Opportunity
• The great list of Principia questions

Early development resources and discussions from when the game was code-named Mathematica:

• Actual Play: In the Company of the Courtesan
• The help me re-name my game discussion
• Mathematica posts on Attacks of Opportunity
• The post that started it all