I only use spies when I can see that my nieghbors have neglected to put up guard towers. Then I get to work trying to discover secret vices of people, which doesn't usually work, I did get to reveal to the world that the King of Italy likes little boys one time though. Spies only work if you only send them to places you know they can get in. A few times they also warned me about impending invasions which was pretty useful, but their reports aren't always true.