The Krell Shield is obtained in the Kitchen, where it is hanging on a wall. To get there, you'll need the Master Key, obtained from Bookshelf, where you have to go through the mirror, turn small, drop it, turn big again, and then return to that same room. Once you have it, you can find a locked door, to the right of the talking tree. This is the Kitchen. Once inside it will be on the wall directly to the right of you when you enter.