I think assassins should be gone. I love defensive units and I personally believe defensive units are ultimately weaker anyways but the game should be about those who rely on defensive units to be able to continually and reasonably defend their city with out the fear of oncoming assassin attacks. I just think assassins ruin the game and if they were gone more players would be willing to leave out their defenses 24 hours instead of hiding them in the Acropolis. It should be a fair battle, your offense vs my defense not my defense vs 2 or 3 assassins. Hope they eventually get banned from the game.
Though, if they have to stay I think it would be cool if assassins could only do damage to a specific unit. That means the person would first have to spy the city and see what is defending it then choose what type of unit the assassin is going to attack. At least that way the assassin can't kill everything in it's path. Players would think twice about sending an assassin if the city is being defended by mediocre units they wouldn't want to waste it. But if the city is being defended by powerful units they would probably use it but can only choose one unit type to attack.