There was a thread about this somewhere recently but a quick scan couldn't find it so no link I'm afraid...

Now I wouldn't normally contradict R'as but I think in this instance he may have overlooked something: I believe ALL agents help to detect enemy agents in your territory, not just spies. It's just that some are better than others (SPY >> ASSASSIN >> EMISSARY). If you have a border fort then this will do all your agent-catching for you, but if if you have no border fort, then your own agents will do it - with the benefit that they gain valour by doing so.

Hence many players will NOT build border forts, so their agents gain valour instead (which I've started doing since I learnt this trick!) I even create honeypots now -- eg as England, I place my king in Scotland - without a port, build a port but no border fort in Northumbria and keep an assassin there. All the enemy agents have to pass through Northumbria to reach my king. Except they don't -- my assassin catches them all and valours up quickly When he's useful enough to send abroad to go killing Popes or whatever, then I change shift and train up another agent. The AI never seems to rumble this little ruse