Till now no one has still used spies against me. But I have used them a lot. I've developed my country so much that I can now use 20-30 spies. The spies steal a lot of money and destroy a lot of resources + they often successfully sabotage buildings and intercept messages.
I'm not against all of these features. But in any country after continuous acts of terror the government takes counter measures. Imagine if my allies (I've 3 allies that are as strong as me) also sent 20-30 spies to sabotage a country that has just 20 provinces. After a week so many spies will destroy the economy of that country and also some buildings. what I'm trying to say is that spies are too strong. Here's what I suggest
1. Introduce martial law that would decrease the probability of spies to succeed.
Side suggestion: martial low may also temporarily reduce the probability of a revolt but gradually decrease the morale in the current region.
2. After the spies succeed in an act of sabotage in a city, temporarily reduce the probability of the spies to succeed especially if there is counter-intelligence.
3. It should take time for the spies to enter a foreign country, so they shouldn't just momentarily appear in a country.
I could say more, but I'm still new to the game (I've been playing for just a month).