It seems to only happen on the south side walls and only when then enemy's are right next to them.
And once again I had to cute bow the screen shot, ito make the file size be small enough to upload on the forums. That picture was taken in the main entrance to the castle on the right side.
1) The Abby has a shelf a player can "step into"
2) There's a patch of grass in NW Sagar that a player can't walk over, but it could be a deliberate obstacle, and if so, maybe a more obvious obstacle like a rock would be more suited?
3) There's a spot in Z. Mountains that a player can step onto the foreground.
if you mean the "sprite mirroring" to save RAM, yeah, it's done that way to reduce RAM usage, needing to have just 5 directions in spritesheets rather than 8. 2D isometrick on mobiles requires compromises like that one, unfortunately.