Meh, one can assume that if an army is even considering using Seige Weaponry they don't have the ability to defeat their opponents in more efficient manner.

I.E: We are not dealing with an army of super-powered wizards.

Again, it would probably be safe to assume that if an army is marching against a position with siege engines you should be relatively sure that a seige engine is going to work. They are expensive, intricate, and therefore very hard to work/get ahold of in a setting where you don't have a super powerful caster doing whatever a super powerful caster might do.

Therefore, we can assume that there is probably also no uber-powerful magic users on the defenses side either.

Of course, this could all be wrong, and you all could be making very valid points if we're talking about a campaign full of level 20 Wizards.

But the issue I see here is that everyone is making the assumption that this man is playing the same way you always do, which is, in all probability, not the case.