OK, so you can see the stream on the outside of the tomb. The floor has partially fallen away on that edge, granting access to the stream via a cleft. The ceiling has also partially collapsed--the debris from this collapse should be illustrated as a pile in the center of the room, but it is not (a mistake on the artist's part).
There is not so much rubble that you can't walk where you please; it is, as I said, concentrated in the middle.