This is really naughty, but if I lose any assassin over 4 stars to something as annoying as a border fort, I consider reloading the game (if there is a very recent autosave). They take so long to build up, and it just boils my blood when they get wasted and they're not even putting the knife into someone in the process.

Of course, if one owns Syria, good assassins are a dime a dozen. If you build a rookery in Syria, you've got VALOUR 5 assassins being spawned like sewer rats.