Very fun! Pretty precise, but nothing unfair. I love how the extra bullets let you navigate some previous areas with ease. I didn't manage to figure out how to get the blue diamond in the room to the left with a grid and four masks (sconces?), but I figure it is some sort of map indicating where the gem will be hidden in another room. However, I didn't get the chance to find out because *SPOILER*
When I found the secret room by hitting the red triangle, the game totally locked. It showed the other room for a moment, then popped back to the previous screen with a slightly faded appearance, as if stuck in a spawning animation, covering the whole thing. No button presses do anything.
Edit 2: replayed, it worked now. But HOLY CRAP that secret level is nasty hard. Way crazier than the rest of the game! Wish it had some checkpoints. Or it does, and I just haven't gotten to one yet.
Edit: Also, KITTY! :D