why didnt european castles have banked walls backed up by earth like japanese ones? this would make castles so much stronger, as the walls wouldnt ever get destroyed by siege equipment.
While waiting for a new answer, you may enjoy these older ones.
/u/ParallelPain and /u/tenkendojo have some answers in this post
/u/bigbluepanda's post here leads to further discussion.
And /u/willOTW's post here discusses construction materials.