By "except walls" it really just means, "creatures with 0 attack power", so any stones, trees, towers, etc. also count, like damping tower, monument to rage, or any of the various campaign creatures like the many stones and altars. It works the same with minotaur commander's boost. There are no "walls" in the game except by namesake, like wall of reflection (illusion creature) or wall of lightning (air creature), you will not find any "wall creatures" anywhere.
...Ah, thanks very much for that. So if a "wall" is any creature with attack=0, then I guess that means that any creature whose attack is *temporarily* zero due to some other effect is also temporarily a wall. That could be a little confusing in a case where a creature simultaneously has its attack reduced by 1 and also increased by 1 due to effects of other cards.
...I'm absolutely loving this game, even if the text seems to lack the extreme legalistic precision that we see in MtG. I guess that's only to be expected given the age of the two and the relative resources being poured into each.