What I feel is a more accurate description of Defense in general within' this game is this:
If you walked up and punched a wall made out of drywall, you would go through with minimal effort. Your attack hits much harder than the defense it has.
If you walked up and punched a brick wall, that wall has a lot of defense. It's going to hurt. A lot.
The key difference here is knowing how much damage you do. If you don't have more attack damage than they do defense. You're going to do almost nothing to it.
It doesn't matter whether we are talking about troops vs troops or talking about troops vs walls.
Siege on the other hand is kind of like taking a sledge hammer with you to hit that wall. While it may not be guaranteed to break through on the first hit, you definitely have a better chance, in my opinion.
If you contact Live support with this matter they more than likely will mention the fact that this player had a lot of defense within' their wall. Or some variation of this, but no matter what, you're going to hear them mention Defense being a/the factor here.
Good luck in the future & happy gaming!