I know I played this at the time it was released, so hopefully I left some comments on IRC. Since then, I made my own heavily abstracted Knytt Stories take on SMB 1-2 in the level Juni Jones: Ace Attorney which was probably subconsciously inspired by this.

Anyway, I really like this level! Some of the underground portion is way too tough for me. The castle being totally solid and mechanically insignificant was one of my favourite parts. The tree in the clouds was another of my favourite parts. The implementation of the flag is the last favourite part I will mention.