The spy is paid in copper. For the espionage to be successful, the client must pay his spy more copper than the amount available in the target castle. If he pays enough, the copper will be used up, but if he does not pay enough, the spy will be arrested and the copper will still be lost.
If the espionage has been successful, the spy will deliver a detailed report about the buildings (incl. upgrade levels), defensive units (including those from other castles) and any resources stored there. However, the spy cannot see any troops that are outside the castle at the time of his espionage mission (e.g. on missions).
Defense against Spies
To fend off an espionage attack, there must be at least as much copper available in the castle as the client paid the spy. In this case, the amount of copper to arrest the spy will be used up. If there is not enough copper in the castle, the espionage is considered successful, and the castle will be unaware that it has been spied on.