How can you tell if units that defect are OK or not?
The computer players will almost always send normal 'peasant' spies against you: combat 10, no skills.Defecting units are almost always kosher if they have skills in leadership, combat, mining, etc. Once in a while a normal peasant will defect, and I find out he was kosher after executing him. Ooops, sorry. But peasants defecting to you are pretty rare; true defectors are usually skilled.
Your spy village:
Now that you know which incoming units are likely spies, have them all settle in a spy village. Apparently enemy spies are under orders to not revolt against you, so go ahead and hit the 'tax' button a few dozen, or -- heck -- a few hundred times. After you're tired of taxing them into the ground, execute them all; if you leave them alone long enough the computer opponents will emigrate them again and you'll lose track of them.
Assuring fort loyalty:
As people have noted, the game does not permit double agents. As far as I know, spies cannot defect or be turned by other spies. (If a spy tries to turn an enemy counterspy, they are discovered.) So, everyone in a position of power should be a spy. Make sure all your generals are spies. An added benefit is that spies seem to have a pretty fixed loyalty -- you can leave a general/spy out standing in the rain forever and he won't defect.
How can you have spy leaders and generals?
Well, generals that you bought from the enemy are naturally spies. So they're good. The other way to train spies as leaders is to settle a small village using the settle button for peasant spies. Then train everyone in the spy village as a leader. Sure, they're only leader-ship 20, but give them a few years in the command of a troop and they'll be fine.
I am not sure that the computer's spies are immune from bribing; I have bought back generals that have defected before. Perhaps they defected not from a bribe but from a spy spreading discontent.
Lastly, always keep your forts FULL. Forts with slots open often find those slots filled quickly with enemy spies ... if you must leave slots open for some reason, check the forts frequently for suspicious leadership/combat-10 characters.
Turning a fort:
As you may have noticed, turning a fort always make the computer player declare war. So if you want the men without getting a war, wait until the general and his men are in the field, and then turn the general. The manual notes that the general's men will usually/often turn with him.
-- Richard Wesson
Be sure to change your spy to sneak, when attempting to infiltrate a human (or computer) controlled village. Sneak does not notify the opposing player of the spy, while surrender does notify him/her.
Surrender should be used when trying to fool an opposing player that the unit has left a neutral city and joined their side. To make this believable the spy should originate from a village of the same type as the spy and the opposing kingdom should have a high reputation score. Make sure you give the spy orders to move to the kingdom's main village as this is how the actual defection works.
I found two very effective ways of inserting spies into foreign forts:
--Vince Indriolo
A spy caught in the act lowers your reputation by three points. If you're reputation is high enough, a small drop will not hurt you (much). However, if compared to your opponents reputation, this loss will enable them to attack you without incurring much of a reputation penalty, be careful in your espionage.
Don't do the crime, if you can't do the time.
Spies in a village. I once captured all the spies sent against me early in the game and settled them near a fort but not close enough to any 'work' areas. Might as well let them provide food, etc. And I noticed that the morale of the troops in the fort (but not the general) was very low. So I trained some of them as my spies and this seemed to work. Apparently the game does not allow double agents. Periodically the game would announce that another spy had been found there and executed. While I don't recommend this -- the morale of the village is very low and lose control of it and the villagers reappear as independents -- I do watch the morale of my forts that are near independent villages.
--Alma Engels
While a (human controlled) kingdom is under attack, attempt to place a spy in one of his/her villages. Human opponents will often be focused on the battle at hand and too busy to check the information screen. If the village is near a copse of woods, place a spy (set to neutral) in the woods and wait for the opportune time.
Camel trains make excellent spotters for games involving the fog of war option. Move a camel near an opponent's village (especially an opponent with a diplomacy level of tense or below) and leave him there. You will be able to keep an eye on your opponent without the risk of losing a spy. The reputation loss for attacking the camel should be large enough to keep your opponent from making camel soup.
Train spies early on in the game. The computer AI and/or your human opponent will most assuredly be doing the same. Place counter-spies in your village, fort and tower of science at least.
Hire spies from the Inn as early and as often as you can afford. Place the spies in independent villages (of the same race as the spy) immediately. If you can manage to place them there before another Kingdom has explored the area, you increase the chance the opposing kingdom will believe the spy is an independent villager. Once placed, the spy will be able to make a believable surrender (if needed) or be in place if the village is absorbed by another kingdom. One added benefit of this strategy, if playing with the fog of war option, is you will be able to see if your opponents try to send a spy near the village and surrender him to your kingdom.
Change the color of a spy to neutral, move him near an independent village (containing a least a few villagers of the same nationality) and "surrender" him to one of your opponents. While the opposing Kingdom may still execute him on sight, surrendering him near one of your villages assures him of a blindfold and cigarette.
Spies make excellent scouts early in the game. With the added benefit of stable loyalty while exploring, the spy can easily settle in a village of his own race, again, perhaps prior to another kingdom exploring the same area.
Add a espionage strategy here.