Tip: Make sure that ALL exit-enabled pipes, doors, etc. have a screen exit defined, or they'll take you to Level 0, the "endless" bonus game. Unless Start+Select is enabled, the only way out of it is to reset the game.
The design is more or less good. There are no particular shenanigans made, though it's nice since it's well varied and lets you explore through many paths, which is nice. Since it's a first world, it's obviously not too hard, but it's not this easy too if you ask me. Some sections are kinda challengy for a world 1. Minor, but I liked how the author hiddens the key in the first level, very smart. Overall, it's an okay hack in design.
Aesthetic-wise, could've been better from my point. I think that some palettes feel a bit off. Mainly that. The GFX style is okay, it doesn't clash and it's most of the times correctly assembled in levels. Not much more to say.
As only issue, I point out that music breaks on accurate emulators.
Overall, a good demo. Something different which works decently.