@Doc von Schmeltwick: As someone who's currently working on a ROM hack of SMB3 (NES), I can tell you that they are in fact not separate. One of the first things I did for my hack was alter the palette of the World 1 map, and upon changing the color of the background (technically the ground since it's in a top-down view), the background color underneath the dancing hills didn't change. After looking at the sprites for the dancing hills in the data, I found that the devs (or artists, I guess) actually filled in the empty space with the color of the ground. No clue why they did this, but they sure as hell did.

Changed the note for the blue dot since it also appears in world 4.

Hmm, I see. So my last guess would be that this is for backwards compatibility reasons. The game's code relies on it having one layer only in this area and modifying it to revolve around two layers could unintentionally break something.

You interact with the layer one frequently, like when you destroy a fortress, animated tiles, etc. I guess Mario's ability to move or not to move in the map relies on layer one's tiles too. Changing that to revolve around two layers could end up unintentionally breaking something in a game that's technically already finished. That would be hard to spot during development too, as they would need to replay the entire game.

The ground being textured on the same layer is probably just a way to improve the graphics without needing to add another whole layer.

But I could be wrong, though. This is just a guess.

On the NES version, they are technically separate, by virtue of the solid color they are on being the transparent background. In this game, the ground is textured, so they can't do that.

I would say that's probably a side-effect of this being a NES port.

It's all on the same layer.

Don't the hills and castles and such exist on a separate layer from the ground behind them? I thought they did, could be wrong.

Added the fortress from world 8 and the blue dots from world 3.