I liked the premise and the gradual unfolding of the levels. Putting the notes to walk over is a nice way of communicating to the player without an intrusive or tedious mechanic.
The two things I'd suggest are - making the player size smaller so it doesn't have collision problems going through doors. The other would be to keep the notes with door combinations close to where they are used so that it's not a matter of memorisation and guesswork. (For myself once I got past the laser on the right side I couldn't open that door despite trying combination after combination.)