Do spies help catch spies?

I'm playing my second full game of BtS and am still getting the hang of a few of the new mechanics, especially Espionage and the Apostolic Palace.

My primary question now is about protecting myself against enemy spying, especially tech stealing. If I build a spy and just leave it sitting in one of my cities, does that help catch enemy spies that come to steal my tech? Is there any other defense against tech stealing other than the counterespionage mission that doubles the cost of missions against you?

Every time I research a new tech, it seems to be stolen right from under me.

Also, can spies sabotage/destroy the Apostolic Palace? (I assume they can't but just making sure, since that's been the other thorn in my side in my current game.)

Yes, spies do help in catching enemy agents, but there is no 100% efficient defence. Or, if you can, build Security Bureaux (which provide the same defence as a resident spy) and put your own spies around any vulnerable borders, in the hope of picking up infiltrators before they've done their dirty work. But even with lots of your own spies about the place, Security Bureaux, counterespionage, and a huge advantage in espionage points, there remains a chance that the enemy will get through.

Bushface said:

Yes, spies do help in catching enemy agents, but there is no 100% efficient defence. Or, if you can, build Security Bureaux (which provide the same defence as a resident spy) and put your own spies around any vulnerable borders, in the hope of picking up infiltrators before they've done their dirty work. But even with lots of your own spies about the place, Security Bureaux, counterespionage, and a huge advantage in espionage points, there remains a chance that the enemy will get through.

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I've never seen the AI use them on civs located on other continents though. If you want to cut down on spy damage, knock the AI off your continent or do the things I'm quoting above.

Spies are seldom more than a mild nuisance unless you make enemies with a large civ who has the EP and hammers to just FLOOD you. That's not advisable in general, though. You want powerful allies, or weak/dead enemies.

If it is just one or two civs that keep stealing your stuff, you may find it worthwhile to keep the counterespionage mission running constantly (the time it takes to expires depends upon game speed), this will (a) increase the chances of catching spies and (b) make it much more expensive for the Ai (in terms of EP) to steal a tech.

You don't have to worry about any wonder, national or world being destroyed by spies. It cannot happen. The closest they can get to sabotaging a wonder is when it is being built, but once it is complete you are safe.

They can however destroy space ship parts already built, and also 'reset' the build time on parts that you are trying to build.

This can really screw things up if you are going for a SS win and it's close.

As TheMeInTeam says I find it doesn't happen often at the levels I play at (monarch), but just finished a game where I won, but only after the dutch caused chaos with spies (I was the Zulus and some early conquests left me behind in money and tech, and William must have been really stacking up the EP's)

3-4 spaceship parts destroyed, my only oil well about six times, and for some odd reason a customs house in one city near the border about 5 times

A spy does increase the chance of catching enemy spies in the tile it is in but there are many other factors to consider. Bhruic wrote this excellent article on Spy Detection which cover all the factors and these include having a spy or Security Bureau in the tile, open borders, the ratio of your total EP spending to their total EP spending, and whether a counter espionage mission is active.

However catching their spies is only part of the problem. That just costs them the 40 hammers a spy and that is trivial in the late game. It also delays the mission until they can build another spy and move it into your territory. But the EP points they have invested against you are not affected even if they were caught trying to steal a tech costing thousands of EPs. Another defence against spy missions is to make the cost of their missions much higher. Two things you can do quickly are to close borders (gets rid of the 20% trade route bonus and increases spy detection) and to run the counter espionage mission which adds 200% and hence triples the cost of their missions for a time. But the best long term defence is to increase your own EP generation. Their missions are cheap and successful because you have not invested enough in the espionage system.

To generate a lot of EPs you don't need to raise the EP slider much but you have to start early by building courthouses and then later jails and other EP generating buildings. If you have a big empire and you build these buildings early they alone can provide an adequate espionage defence and allow you to get the passive bonuses and maybe run a few cheap missions yourself. But you do need to research or trade for the relevant technologies (CoL, Constitution, Democracy and Communism) early and then build the espionage infrastructure fairly quickly. If you have spare food in a city run the spy specialists as they are a very cost effective way to generate EPs

If you spot an AI building up a large espionage total against you then you can counter this to some extent by raising the espionage slider although this costs you gold or beakers. I often raise the slider just for a short time when I first meet a new AI in the middle game and concentrate all my EPs against him. That lets me see what he's researching in a couple of turns and let's me decide what to trade with him or his rivals. Those investments aren't wasted as every EP point generated helps improve the spy defence and might be useful if I decide to mount spy operations against him. In the meantime I get valuable information that can be just as important as the research or gold I lost in gaining it.

Thanks everyone for your responses, they were extremely helpful. The resource management aspects in BtS are definitely challenging. There are so many more factors to consider vs. vanilla/Warlords and more hard choices to make since you cannot build/have everything.