Spies 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, which is why it is usually not affordable. However, in the late game when all but one team are almost finished off, Spies usually becomes affordable and helps speeding up the finish of the game. Presumably, this was the intention of the developers to include Spies in the first place, as without it, games on larger maps can end in long and pointless scouting missions.
It is disabled in Deathmatch mode games due to the massive amount of starting gold each player has, and in Regicide games, where it is replaced by Treason.
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.