Spies are really getting out of hand.

[Kraxis]

Does anyone know if it helps to have a mix of spies and assassins in your city, to root out enemy agents? Or is it just the sum total of subterfuge points for your agents in the city, and it doesn't matter if it's spies or assassins?

One other thing I've noticed is that castles are fairly immune to infiltration. I think maybe the campaign AI doesn't bother sending spies there to cause unrest, because of the built-in resistance with castles. It means I can focus more on protecting my cities.

The AI does infiltrate castles. But you just don't have many of your own spies there (for obvious reasons).

You should notice when the AI assaults castles. Sometimes the gates are open. And one of my elite spies in Dijon managed to watch as the French infiltrated Bern... So it happens.

and if the spy that's troubling you is just ejected and not killed, you can target him with your assassins.

To be honest I seldomly get any good chances of spies. Like a level 7-10 assassin getting about 15% chance against most spies. So I simply don't try, it is not worth it. Besides that spy should be dead next time he tries to infiltrate (they never do seem to pick new targets).