Turtle Valley: 0F72-0000-0029-44B5
A few days into making levels, while I was still waiting for all the tools to unlock, I discovered that hopping Parakoopas will pretty reliably bounce three spaces forward and one block up. That was the observation that inspired the first third of the bottom route through Turtle Valley. There are two main ways through: the bottom you're pestered by Green Parakoopas and Goombas, the top Red Parakoopas, Fiery Pirhanas, and Flying Spinys for good measure. It's probably a bit too crowded with enemies honestly. Oh well. The top route is much easier than the bottom, despite having all the jumps. And: there's a big secret area near the end.
Note 1: Green Parakoopas are my favorite enemy. They're comical, but also challenging. You can tell the poor Koopa doesn't have the hang of those wings. (For more on the plight of the video game minion, I refer you to Homestar Runner's Brothers Chaps and their "Two More Eggs" cartoons starring Eggpo: New Job and Waiting.)
Note 2: It's fun to, frequently, have a way to gain a random Mystery Mushroom near the exit. So many costumes have their own clear music, taken from their original games, that it feels good to showcase that. This is extra fun to provide at the end of Castle levels, if you can find a way to do it without upsetting the desired challenge level, because some costumes make Toad or Peach say something different if you beat 100 Mario Challenge with them! BTW, the last level in 100 Mario Challenge seems always to be a castle. (Or, once in a while, maybe an Airship...?) Anyway, play around with Mystery Mushroom costumes when you get one. Some have special noises they play when you jump, land from jumps, or even just idle (like the Mario Kart costume). Most also have special music or sounds that play if you fall off the level or get crushed while wearing them!