Tip: After editing your overworld, make sure to use an EMPTY save file, as save files with progress may load Mario's position based on previous versions of the overworld. Note that altered level flags will also only take effect on new save files.
I think you´ve got a nice difficulty curve and I like the thing with the trampoline and Koopa near the end. You´ve done pretty well with the few pieces you´ve got, and the level doesn´t overstay it´s welcome. I´ll look forward to what you can do in the future!