No, of course it doesn't actually have to have 2 floor levels. My point was that if you're going to pretend to interface with the tiles beside you on those extra floors, they have to be roughly the same height, that's all. At least one of your images had two distinct levels, even though one was partial, that just didn't work at all with the vents. That said, I would fix them if you could demonstrate why you say it's such a minor thing. And just cutting out the space you're working on and saving it as a new image would do that job just fine.