Thanks so much for youe indepth critique! So much useful information! Time to sit down and fix some visibility issue  :P

The contrast is very low, so it's hard to tell at a glance where the interactive areas (walls, floors, ceilings) are. It's good to keep the background contrast a bit low so that the player character can stand out, but you also need to make the background read clearly. If I had to fight enemies or do anything else that stopped me from being able to carefully pay attention to the tiles while navigating a room with these tiles, I'd likely run into walls or fall off ledges. Playable surfaces have to stand out so that the player can easily see them.

Fortunately, the non-navigable areas also have the benefit of usually not having characters or important objects directly  overlapping them, which means you can get away with higher contrast there than you can on the background walls, which are overlapped by characters.
 

Everything feels a bit flat. The shadows from the platforms help, but they aren't enough. Again, they're a bit too low-contrast, and they're only below the objects. I think it would help a lot to have tiles for shadows cast to the side, so that the different platforms can pop from the background wall. Making the platforms (small-bricked areas) a little lighter might also help create both a little volume and better readability for what's a wall and what isn't.

I think the column tiles should include small shadows with them, so that they look like they stick out from the wall, but not as much as the walls, so that they still read as background elements.