Hi serpent - you stand more chance of losing your assassins if the other faction has spies, assassins or other strategic agents in the province - seems to help the border forts no end. And you can't see them so you don't know 'til it's too late (unless you save your game and do a faction swap cheat, I guess). I find I catch a lot of spies and assassins when I have my own in the provinces with the border forts, so I guess that's how the AI gets mine, as well!.