Onagers

Hehe, had this experience yesterday: while a group of my hastati were pressing hard for the city square another one was holding off wall defenders so they could not reinforce the square. Suddenly, I start hearing explosions near where the wall fight was happening (I did not bring any arty with me). To my amazement, the AI had fielded two onagers at the end of the street and was pounding right into the middle of the fight... Decimated both: AI and my units almost to the last man and destroyed a few buildings while at it... Funny.

Yes, they'll use the onagers inside the city walls to blast away at anything resembling your army, no matter if their own guys are taking losses.

I had one battle seiging Carthage where they put two onagers on the street about a hundred metres away from the gate and blasted anything that came through. My poor peasants...