I wrote a review of this game a several weeks ago, and now you can download it for free if you have an Xbox Live Gold membership! This game will only be free for about a week, so check before this post pays out!
I haven't yet completed the game at 100%, but (spoiler alert) I have now explored most of the inverted castle. I also got lucky and acquired Crissaegrim via its random item drop, arguably the best sword in the game. I am still trying to get the magic attack system right though.