Once you have researched Spy Education and have trained your spies to a level you see fit, you can either use them to run spy missions, or keep them at home to protect your city from enemy spies. Similar to mages, there are two kinds of operations you can send your spies on - info gathering ops, or stealing missions. If your spy successfully completes a mission, it will return in the number of hours specified on that kingdom's overview page (except Huginn and Muninn, who always take 12 hours); if your spy fails, it will be captured by the enemy city. When spies are captured, their return time now depends on how "close" they were to succeeding. If they were really close, they will return in the same amount of time as if they had succeeded. If they didn't come anywhere close to succeeding, they will take the maximum 18 hours to return.
Information Gathering Operations
These four missions are available to all cities as soon as they leave the guard. If you choose not to use your mages for gathering information, then you will always need a Barracks Spy (BS) and City Spy (CS) to determine an opposing city's full defensive power.
Retrieves basic information about the targeted city's hero and detailed information about the targeted city's troops.
Retrieves detailed information about the targeted city's resources, buildings and land.
Retrieves detailed information about the targeted city's hero attributes and equipment (both equipped and unequipped).
Views the news of the kingdom that the targeted city is in.
Resource Stealing Missions
There are a total of 10 stealing missions available in Norron. Like self spells for mages, these missions are locked at the beginning of the game and must be researched to become available. The higher your spy level and the more of a resource a city has stockpiled, the more resources the spy will steal on a successful mission. Once more complex missions are unlocked, the lower level missions will no longer be available to choose from on the Spy Mission page.
Steal Iron Ore
Steal Food & Lumber
Steal Lumber & Iron Ore
Steal Food & Silver
Steal Food, Lumber & Iron Ore
Steal Lumber, Iron Ore & Silver
Steal Food, Lumber, Iron Ore & Silver
It is possible to capture opposing cities' spies, and have yours caught, through the use of spy defense. By leaving your spies at home, and not in training, they automatically help protect against spies attempting to get info on your city or steal from you. The rough estimate is that every 10.5 spy levels you have at home, accounts for about 1 level of spy defense (through Vhal's experimentation). So if you have all 20 of your spies at level 51, and sitting idle at home, you will have a spy defense that will stop most spies level 100 or lower (20*51 = 1050/10.5 = 100). Note that information ops are easier to succeed with than stealing missions.