It's not too bad, but there's that empty space at the start, a few areas that feels cramped (especially the part with the ropes), a small lack of coins, and uh, what's with the pointless star in the middle of a zone with nearly no enemies? And there's also the two pointless invisible block around the block containing the star.

A bit later, you find a red pipe right on the path, absolutely visible (unlike the one in the original castle) within an area with absolutely no pipes. Like, just by looking at this pipe, you can guess it leads to area 3.
Said area is fine, but if I were you, I would have put the star nearly right in the corner, instead of next to the two blocks.

When you get out, you can barely escape from the 2 blocks tall space you have to groundpound in. Many people would end up stuck in there, you shoud consider that.

Then next, the wall next to the Boss door has got the wrong tiles. Those are meant to be used between the wall and cielling tiles.

In the area with paratroopas, you're going to be in trouble if you don't have a fire flower.

Bowser boss is unchanged, and should remain that way.