Secret room in the library after you defeat Sweetheart.
Guide to Secret Room
You start out here once you fall down the well. What you need to do is read all of the excerpts of the books in the library. There are 6 different books pertaining to:
- Tree house
Once you read them all then the room will unlock. It is located through this beam of light. If you come here before unlocking it then it will just indicate that the wall appears hollow.
A piano, save point, and fruit is located in the room. I suspect that there could be a way to further progress here. The back wall of the room has 3 hollow points. However, I could just be making assumptions. It could just a fun room with no way to further progress.