I like GeneralHankerchief and econ21's ideas. I'll second them. Spies tend to live a long time, but assassins tend to die really fast however. We might say we will not make the assasination attempt if we have less than 50 % chance of success or something like that. Then there is the real possibility of getting assasinated. In my last campaign I lost four generals to that, two of which were my very best. Especially Anatolia (Turkey) can be really nasty as it is a sort of crossroads. I remember a shot where my general was in a city with 10 (!) assassins around it. That didn't end well. I advise sending spies with capable generals. That helps to keep the number of assassinations down.