Several 3-up moons all next to each other = bad idea.
Making paths out of water tiles = bad idea.
Not using end tiles for ledges = bad idea.
The level design in general fails. Play through some of the featured hacks, you could learn a lot from them.
And why is there so much damn blue?
EDIT: While I was typing that, the "Don't remake levels" tip was shown at the top.
Alright, I'm going to go through each screenshot and tell you what you did wrong.
Chasm of Chaos
Screen 1- First of all, the white background, blue sky and brown foreground look horrible. Those jumping note blocks are in a really awkward place, too. Putting them right on top of a ledge looks weird.
Screen 2- Looks like you just stuck random blocks all over the place. Coin blocks make really bad walls/platforms, and you have 3 extra blocks sticking out that look a little weird.
Screen 3- ...What? So you did just stick random blocks in completely inappropriate places. What are those green switch blocks there fore? Also, put an end tile on your ledge.
Screen 4- Again, why is that green switch block there? Why are there icy blue turn tiles and grey cement blocks garbled together to make platforms? Why are there 3 3-up moons next to each other? Why did you use a green star block as an end for the ground?
Screen 5- Looks bland. Avoid long, straight, boring platforms.
Screen 6- Those water tiles look like shit. Fix them
Screen 7- Not bad, I guess, but those better be unbreakable turn blocks.
Screen 8- Very bad idea. Besides, if you're going to remake an extremely popular level, don't add a note saying it's a remake of an extremely popular level.
Plains in Peril
Screen 1- Half of the chimney is blue. If you're going to do a palette edit, don't just change random colors to blue.
Screen 2- Nothing to say here.
Screen 3- Not bad, but it looks a bit bland. If you're going to put those slopes sticking out, put a sliding shell-less koopa on there or something.
Screen 4- That looks really, REALLY bad. Don't use question blocks- or anything that isn't completely solid, for that matter- to border water.
Screen 5- This is even worse. With those coins there, the water is just floating on air for no reason. You just can't use water that freely without making it look weird.
Screen 6- Don't make really long, flat areas. They're boring.
Screen 7- Another reason changing random colours on a palette is a bad idea. The corners of those blocks are blue, but the rest is brown.
Screen 8- Don't put stuff right over dirt tiles like that. Put a ledge under those black piranha plants.
Screen 9- Put an edge tile on that ledge.
Screen 10- Not too bad of an idea. Why is there a dirt tile sitting next to a ledge? If that's supposed to look good, it doesn't.
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Mario's Nightmare Thread