I just played and beaten all of your levels. Overall, yeah, I can see why they have low clear rates, and it's because you have many players who can't even get past a simple Goomba, so beating something, like the bullet hell-like Halberd level of yours, may be downright impossible to many of them.
That said, I very much enjoyed them. I didn't see why you have a Rail Castle 2.0, since there's hardly a difference, other than some aesthetic changes. Overall, I enjoyed them, and if there's any advice I have for any of your future levels, try bringing back the difficulty a bit. Make it fun and beatable for the vast majority. It's a different story if you made a level hard by design, with the target demographic being pure experts and Kaizo players, like with my "Bounce: 'Prepare to Die' Edition" (which I've since deleted off the server to make room for my more normal worlds). Anyway, good job.