|This article is about the technology in Age of Empires II. For other uses of the term, see Spies.|
|Introduced in||The Age of Kings|
|Research time||1 seconds|
|Gold||200 per enemy Villager|
|Effect||See enemy's Line of Sight|
Spies is a technology in Age of Empires II that can be researched at the Castle once the Imperial Age is researched. Once researched, it allows the player to see the enemy's Line of Sight. Spies is the only technology in the game to have a variable cost. The cost depends on how many Villagers, Trade Carts, Fishing Ships, and Transport Ships the enemy (including 'neutral' players) team has. Each of those units increases the cost by 200 gold, with a minimum cost of 200 gold and a maximum cost of 30,000 gold. Spies are researched almost instantly.
It is considered to be one of the most useful technologies as it grants players a permanent view of enemy movements. However, Spies is substantially the most costly technology to research, more so in games with high population caps. Players wishing to research Spies are recommended to focus on mass gold mining and extensive Market trading.
A good time to research Spies is when a team has killed most enemy Villagers and is trying to finish them off. Being able to see their location reduces the effort and time of ending the game when opponents aren't resigning.
Presumably, avoiding long scouting missions towards the end of a decided game, especially on very large and giant maps, was the intention of the developers to include this technology.
Spies is disabled in Deathmatch mode due to the massive amount of starting gold each player has.
Advanced civilizations of any age gathered information about potential enemies as part of their foreign policy. Much of this intelligence was gathered overtly through the normal channels of trade and diplomacy. Prudent civilizations with hostile neighbors actively enlisted spies and informants to monitor enemy activity. During the Middle Ages the Byzantines and Mongols were especially active in employing networks of agents among their enemies. Spies gave early warning of enemy forces marshalling for attack or searched for weaknesses in enemy defenses.