Mahou Shoujo Tellis-Chan

[Vipershark]

I feel like the walls are too plain, and therefore look out of place.
The way that they're undetailed as they are, they look almost like untextured lowpoly 3D objects (like in starfox or something) while everything else looks cartoony so they look out of place. I think that's what was causing the tree above to look so off, because the wall takes up so much of the screen that it looks like you have two clashing visual styles going on.