Let me clarify something that might help.
There are non-destructible props and destructible props. Typically, the destructible ones are those with the "mm" prefix (meaning Mount and Musket, meaning developed specifically for this module) are the ones that can be destroyed during battle, whether from artillery/explosive crates or from swords/bayonets.
"Earth wall", whether forms a,a2,b, or b2, are non-destructible. The only difference between them is aesthetic; you cannot change from one to the other in gameplay. Correct me if I'm wrong, but those are from native Warband where no (or few props, like doors? not sure) can be destroyed during battle.
It's not always straightforward what blows up and what doesn't. Therefore, I'd suggest taking some time to drop some props down in an open field and experimenting which ones get damaged or destroyed from explosive crates. That's what I did while making my first map.