Thunderbrd
C2C War Dog
But won't city defense buildings damage them? Such as Bombard Towers? Even if they are not engaging combat, standing next to a city will hurt you.
As for "in front" it may be true for a Battering Ram but a Trebuchet would be near the back of the invasion. Possibly out of the range of the defending Archers.
Those methods of Bombardment mechanism are not bombardment but rather are applying a sort of 'terrain damage' in effect. Those city adjacent 'attacks' can and do damage the lead units just fine. What we're talking about there is the bombardment abilities of units themselves (like catapults for example.) Currently they will never damage the lead unit of the stack they bombard.