Counter espionage

[Kaiser Nonsense]

Bloody hell, those bloody prat spies piss me off, they keep causing unrest in my cities, and there is nothing I can do about it! Or is there?

Is there any way to catch them, or keep them away in the first place?

[Avatar]

Stick your own spies in your cities, they are good at spotting other agents. If you get extra lucky your generals can also get counter-espionage traits if they are successful in killing a few spies.

[Duncan III]

Ideally, I like to have a spy in every city and castle for just this counter espionage role.

[Vipman]

Put your own spies in your cities/castles. Enemy spies will be killed or kicked out from your settlements in a few turns and they will have small chances of entering again (like 5%) if you keep the spies inside. And make some assassins to help too.

[Sitalkes]

Also, put some spies and assasins at likely border choke points (e.g. bridges) where enemy spies are likely to travel. That way you can detect them with your spies and kill them with the assasins before they get into your cities. You can even prevent them from entering that area entirely simply by putting a unit or agent in the choke point. You can use lookout towers instead of spies for this purpose.

[Vipman]

I think towers show enemy spies and assassins randomly or when they're close or sitting on the tower.

Agents do no block choke points/bridges, only armies.