I actually never got to fully understand what the cave was. Somehow it always made me think it was a place under an old tree, under its roots, somewhere not man-made. On the other hand it looks like a structure (with no entrance but a hole under a tree?), but never like in Return of the King when Aragorn recruits the dead. I think it's mostly because, basically it looks like a corridor, not like a cave or room. Infact I always wondered what was on the other side of the corridor... If Luke entered by one side, he went out by the other side?