How many spies does it take to change a light bulb!?

[Martok]

Spies are best used in your own faction. Some of the functions are keeping the peace in the troublesome provinces(in fact, 1 spy per province is a must have), or charging of treason to your disloyal generals.

If you want to use it agains the enemies, stay away from provinces with border forts: your spies and assasins will be killed almost always in such provinces, and even if they don't, they won't last for more than... 5 years (believe me, i lost a 7 star assasin that way).

Of course, if there is no border fort, or you really want to try it, do as the above poster says and spam an insane amount of agents, one of them should survive... perhaps.

In my opinion, it's a waste of resources. Forget about the money, you waste a turn that could've been used to train more reliable agents—alims or inquisitors, anyone?—, or an army. But you may not think so...

I concur. As a general rule, I only use spies in my own territory. They're good for improving a province's loyalty and for convicting disloyal generals, but that's about it. The only other use I've ever had for them is that on certain occasions, they've managed to open up a castle's gates so I don't lose men having to assault it. This is a very rare occurence in my experience, however.

[Roark]

I love using spies.

If you've got a decent Navy, or plenty of emissaries / religious agents, it's easy to see which provinces have the security buildings. Avoid them. There are usually plenty of provinces which DON'T have a watchtower or borderfort.

I generally use spies to flood an opponent's empire just before I make an assault on him. They usually serve to tip the unlucky faction over the edge into civil war for me, once I've taken 2 or 3 provinces. This makes further invasion much easier.

Also, if I see a fairly rebellious province with a smallish garrison, I'll plonk a spy in there to try to foment an uprising.

[The Darkhorn]

I concur. As a general rule, I only use spies in my own territory. They're good for improving a province's loyalty and for convicting disloyal generals, but that's about it. The only other use I've ever had for them is that on certain occasions, they've managed to open up a castle's gates so I don't lose men having to assault it. This is a very rare occurence in my experience, however.

I always forget to try this. I'VE NEVER EVEN ATTEMPTED IT.

[Martok]

I always forget to try this. I'VE NEVER EVEN ATTEMPTED IT.

You mean using spies to open up a castle gate that you have besieged? It's quite fun--at least when it works, which isn't often (in my experience, anyway). If you're going to try it, however, there's a few things you probably want to keep in mind:

1.) The more stars a spy has, the better his chances. I know that's a "duh" statement; but in this particular case it bears emphasizing. Of the few times a spy has successfully opened up the gates for me, they all had at least 1 or 2 stars; I have yet to see a 0-star spy pull this off. I'm sure it's still possible for them, but I haven't yet seem them do so successfully.

2.) The smaller the castle, the greater your chance of success. Spies seem to have a much higher chance of opening gates to forts and keeps than they do with citadels and fortresses.

3.) The smaller the garrison, the higher your spy's chances are of getting the gate open. So sometimes you may want to siege the castle for 1-2 turns first *before* sending in Mr. Sneaky to do his thing.

[Knight Templar]

Read this topic
https://forums.totalwar.org/vb/showthread.php?t=52284

I use spies mostly in enemy provinces (of course, avoiding BF). I also tend to put at least one spy in every my province with port.

[Marquis de Said]

The only other use I've ever had for them is that on certain occasions, they've managed to open up a castle's gates so I don't lose men having to assault it. This is a very rare occurence in my experience, however.

I manage to open castle gates quite often. The trick is to wait for a while. It is especially good to use spies for opening castle gates if you want to keep all the buildings in a province, as none will be lost if the spy succeeds.

Here's the trick. Annihilate the enemy army so that it's down to very few survivors who retreat into the keep/castle/citadel/fortress. Besiege them for a few turns. Send your spies to the povince. That way they'll keep it happy while you wait. Every turn, test the castle with one spy to see the odds of success. Once the percentage is high enough (>50%) order a spy or two to open the castle gates. This may consume a few spies, but it may spare you up to 30-40 years' worth of buildings. And if a valour 0 spy succeeds in opening a citadel, he may even get two stars for his troubles. Well worth it in my experience.