Espionage is when you use spies to see either what troops and tools the target has in the their castle or resources the stored there(military espionage for the enemies troops/tools and economic espionage to see the resources of the enemies). This allows players to better prepare attack forces by planning amounts of troops and the proper tools to use. You can increase your number of spies by building/upgrading your Tavern. You cannot spy for economic espionage the NPC enemies. The accuracy and risk of the espionage depend on the number of spies and the detail level set. The higher the accuracy, the greater the risk. Changes of being caught increases with the number of guardhouses owned by the enemy target. You can also do research to speed up the spies, and even add more spies as well.