There's a % above the city name, I think. That's the defensive bonus units inside the city get. If you don't see anything like that, it means the city has no defensive bonus. That number will go down with every turn you use catapults on it. If you stop, or if it gets down to nothing, one turn later the number will start creeping back up again. It will take 5-10 turns I believe to get back to where it was originally.