Board Thread:General Discussion/@comment-27136653-20180228205701/@comment-27136653-20180331130436

My guess would be: Anything built with magic (e.g. monsters' hands) is infused with magic. Ergo, attacking any such object with magic destroys the integrity of that object, since it damages the magic that holds it together (if the magic wasn't what was holding it together, then it wouldn't fall apart once attacked with magic).

This implies that anything the monsters build requires a tiny bit less of the building material or so, as the rest is magic.