Just a note that A4 works the way it does because it's also used for stage tops, cliff tops - basically layering of any kind using the A4 tiles.
The wall is supposed to be the stopping point for ceilings, so that you don't get on them, which is why they're blocked from all sides bar the bottom, where usually stairs (walkable) would allow you to reach the top, and why the other directions from the tops are blocked off (so you can't step off them and onto the lower level from there unless you climb stairs/ladder/etc from the wall direction.
Granted, people don't use them always like this, but yeah, that was the intended use for those tiles - a double-up to save space by having ceilings and cliff-tops share the same tile setting and passability. Because the designers probably thought "Who would be silly enough not to put a wall under a ceiling?" XDDD