I'm a just copy paste the stuff that I said from my 12 Favorite Medium Demons video:

Formally was my favorite demon level overall before Well Rested took its place. The pure concept of a MetroidVania like level in GD. Multiple paths to take, outcomes so incredibly different from one another. And this was before we had the tools necessary to pull it off. This level... is incredibly unique. As I said, there are multiple paths to take, and the outcomes are so incredibly different from one another. Either you are playing a medium demon or an insane demon. The difficulty of the level varies depending on what path you are taking. I really think the gameplay excels in this level because of it. It's standard path is what makes it a Medium Demon though, and I don't blame you for going for this path first. The other paths are hard. Decoration into account, the parts are very different in terms of artstyle, but it compliments the atmosphere very well based on what area you are currently in. And... also my biases towards castle themed level. I've established them once before, I'll say it for the sake of this review, castle themed levels really suit my fancy, and this level very much solidifies it.

Now to conclude it off, CastleMania is quite the unique level. One that I really, really, REALLY like. And I think it is something that really can't be replicated again because of us now having the tools to do so.