Looks great so far on 5-1 and 5-3. For some reason, it reminds me of Pop Tarts, specifically the chocolate kind. I have no idea why, I'm probably just hungry. My only complaint is that the the mountains on 5-3 could be a tad bit lighter, just so it doesn't get in the way of the foreground.
The hack is looking fine. Probably the reason why no one is posting is just it doesn't look fresh to a lot of people. That's just my assumption. It looks fine, and I wish for you to keep on going. Don't let the lack of posts get to you. There's a lot here on SMW Central to look over.
Don't worry if nobody replies on your thread. I know it was said before, but majority of users only post when something you made has some problems or looks simply astounding. Neither of these things have happened yet. The reasons why I'm posting this are explanation on why nobody posts and 5-Castle.
Speaking of the last level, it's pretty nice that rooms have different colors and the boss fight has stones in three different colors and the background in a different colors. I think that you should make the backgrounds in other sublevels slightly darker, so they don't blend too much with those stones. Never mind, they're looking okay. Keep up the good work!
While the idea behind the 6th world theme, terrain and visuals seem interesting, I have a little gripe about the water area, the last screenshot of 6-1, and the first, third and the last screenshots of 6-2 looking empty. As for the overworld map, I'd like some more decoration on the sea or more islands and as for the levels, some moments seem to be devoid of monsters.
Looks good, but it looks weird that you recolored the cave tileset for the "beach" foreground. It's a really good idea to make levels while it's raining, but why are the thunder clouds just squares instead of looking like actual clouds?