I have a theory...

...but it seems like it will be so tedious to test, I'd like to find out in advance if I'm barking up the wrong tree. This has to do with a puzzle from later in the game (at least I hope it is later) so don't look if you haven't gotten pretty far, or are worried about getting spoiled.

I think >!that maybe I need to actually spell out the word CASTLE with the chess pieces by drafting the associated rooms in the positions that contain those letters, based on the picture puzzle, then placing those pieces on the board in the order of said letters to spell out CASTLE.!< I just don't think I could stomach the disappointment of going through that whole rigamarole and having it do nothing.