Do you mean if the catapults damage the wall when hitting?
They do damage, but not very much for some strange reason, they do like 1% per hit or something, and also when playing as Isengard you can order your men/uruk-hai to attack the wall since even though it is ambient it still belongs to Rohan..
Edit: Or not, you can't order them to attack after all, which sucks, cause then you possibly can't script them to either
Edit again: Or nvm, you can attack the wall with catapults, silly me had ordered ballistas too, which cant attack the wall