If you are new to the game then let me educate you on what I believe to be one very simple reality of the game of Grepolis. Spying, to me, is a fools game in Grepolis. No one should ever attempt it in this game. There are many reasons why and so I will attempt to explain some of them. If you can add to this with your own reasons please do so.
1. You cannot be a sideline spy in this game. Most spies are not members of command or soldiers in the military. Instead, most spies are civilians who are not subject to orders and controls from others. The small business owner by the bay who photographs passing ships going into and out of harbor is in a good place to spy. He is fixed, not under military or government control, and has a valid reason to stay on station permanently.
This game does not provide that opportunity. All members of an alliance must contribute or risk destruction from other allies, as well as enemies to the alliance. So, to spy in this game requires active participation in missions against enemies while attempting to help the enemy at the same time. The spy cannot be a passive observer except in an MRA alliance that allows players to just sit by and not participate while broadcasting all plans to both academy and regular alliance members via shared internal forums. However, any MRA alliance who would broadcast these plans is usually not a threat because the reason they are broadcasting them is a lack of active participation outside of full broadcast messages. This means that plans of this sort are almost always a day late and a dollar short and the end result of which usually ineffective against even a medium sized alliance of active participants.
2. Information obtained by a spy in this game is only useful for immediate defense or attack and is therefore highly perishable and of limited value. Almost all information needed to play the game can be gathered by other means other than a spy, such as attack alarms, probing messages, grepointel information, etc. It is this other information that is more valuable than spy intel usually because this other data must coincide with spy intel in order for alliance wide decisions to be made in confidence, which means that the spy intel is largely irrelevant and not needed.
3. All information and contact given to a spy can be used against the alliance that uses the spy. When this happens, the alliance that was foolish enough to believe the double agent looks to be fools for trusting in an enemies provided intel and can be setup to ruin their own attack and defense ability if they base any decisions solely in the possible false spy intel, see number 2.
4. There is no certain way to maintain a spies loyalty. In real life much can be used to hold over a spy, but even in real life, this is little of a threat as the spy has already proven to be of a mentality where little else matters tot he spy, not even their own life, such that what can you hold over one who is faced with certain death daily in the performance of ones own duties. There is nothing that you can hold over an enemy spy to ensure cooperation and as such most spies in this game are solely self serving turning coat to whatever team proves to be the strongest and providing intel to either side in order to determine which side that will be. So, the spy in this game usually works against both sides for their own benefit to thereby join whatever side remains stronger in the end.
5. In order for a spy to be successful for any length of time he or she needs to have a decent intelligence and a lot of experience in communication and spying in order to be of any real use to who uses them. This intelligence and communication experience can almost certainly be more usefully used outside of spying efforts and in regular communications with other alliances for pacts, regular intel, and misinformation. In essence, the same intel can usually be had if one is very experienced in game communication without spying at all, but just through superior skill and expert diplomacy and communications.
In conclusion, in my opinion, spying is a fools game because you are risking much for little to no actual benefit that cannot be gotten by other regular means. It is simply an equation not worth bothering with in this game to me.