I've played it twice just in case...
It's not a game... Games have goals, rules etc... What's the goal of this game? To finish it?... To see what creator put at the end?
Actually ok... it has rules for the stone throwing part which is probably a game element. But what's the point of it? It doesn't matter how good your throw was. It doesn't matter what your score was. It's just like a very weird action to turn a page in the book. (+ you can't grab it right away and it's hard to throw it right at times. Or maybe it's just me)
I understand that you're trying to be deep and I'm all for creating emotions but this one is more like "I've put some art, some sad music, sad topic and text so this is deep".
SPOILER(I guess). When they reveal the material of the castle I feel like it supposed to hit me or something.... Castle of glass... ok glass... So what? Why glass? Does it have any importance?... END OF SPOILER
I guess I could call it a game if I would call visual novel a game but I think it's more accurate to say that it has game elements.