If you attack the crew of a ram they'll drop the ram immediately. You can then retreat and they won't pick it back up again. If they have another ram they'll bring it forward and you can do the same thing until you've littered the gate area with rams. About 10% of the time if the AI can somehow find it's way through the first level anyway it'll go back and try to get one of those rams again, but usually they just stand around the walls getting shot.

As I said, you don't have to defend much to protect a Citadel, but sometimes you have to do a little. Often a single fast company of cavalry is enough, just bump the ram crews as they come. I have never seen the AI make it to the third ring, much less break into it. I actually had them knock down the gate to the second ring once when I intentionally didn't defend the first ring... but then their whole army just sort of stopped and stood around waiting to die.

It's being gracious to call this a bug, IMHO this is an example of the game simply not being finished. Obviously AI citadel assaults in vanilla were simply not tested and have never worked, and even good player mods have trouble making them effective.