Attacking Battering Rams with cities + archers will take off more than half its health. You need a large attacking force to take cities with rams, but once you have one, they make it a certainty.
Unfortunately, Attila has no way to counter horsemen, since his UU replaces the Spearman, so if he doesn't have iron, he has to use warriors until Civil Service. Bleh.
Hwach'as retaining their city attack bonus, on the other hand, is a whole different level. Rams are completely useless at anything but attacking cities, but Hwach'as can attack anything, and Hwach'as keeping their catapult bonuses will be almost twice as powerful vs. cities as a battering ram, and will have the capability to attack units as well.