Just what is it in the tree on Dagobah !?

In The Empire Strikes Back there's a scene where Luke Skywalker decends inside a tree to face an apparition of Darth Vader. What's always puzzled me is there seems to be a man made wall down there, like a corridor,maybe part of a temple, yet I've never seen or read any sort of explanation as to what it is or if it was a scene/idea that was cut early on from the script ! The wall can be seen in these behind the scenes photos from the filming of the scene.