Know when to use them or you will lose. The cost of the spies technology is dependant upon how many enemy units exist. This prevents you from getting spies too early in the game. Spies is usually used to find the enemy's remaining villagers or buildings. There is not a spy unit you can make.
Spies is only a technology gotten at the castle.
Once you click on it you can see everything your enemy can. There is not research time. Yes, if you build a castle and have enough resources, you can imperial. For example, building a castle is the same as building a monastery and siege workshop to meet the requirement of two castle age buildings to imperial. The castle adds twenty to your population count, giving you room to build twenty more villagers or units.