The locked door?
 

In the part of Aragorn traveling through the paths of the dead, it is said that there was a locked door and a armor-clad skeleton lying as if it was clawing at the door.

I can reasonably assume that the skeleton was in actuality the *can't remember name* who tried to ride through the paths of the dead, even though the old man at the gate said
But what was the door?

I remember there was another thread speculating about this...

*searches.....
Here.
(oops, sorry for the red letters of doom...)