So I tried qPerfectionWeld. Pretty cool. Nothing moved but my castle jumped out of the hill it’s half in and rolled off the map 
. I might just have to redo it without terrain touching it at all.
I’m guessing that parts sitting on top of each other will always just clip a little bit due to minimum collision distance? i.e. the height of a wall of 10 unnchored parts will not be the sum of the individual part heights (even if the bottom is anchored), it’ll be just a smidge less. This is pretty annoying since the wall will instantly be in motion, bounce a bit and probably fall over.
I noticed an interesting (if old) thread here that seems to highlight some of the clipping issues: Unlock scaling of all BaseParts under 0.2
If this is expected then that’s fine. Just need someone to say that the physics just works that way. i.e. vertical positioning is unreliable by design unless anchored/welded.