I believe the door was held shut by the pressure of the surrounding rock, and that the priest knew some way of releasing it.
We entered a bare cavern which was apparently an exact cube of about forty feet.

It was the only cavern in all that system of caverns whose walls, corners, roof and floor were all exactly smooth.

It contained no furniture of any kind.
But exactly in the middle of the floor, with hands and feet pointing to the four corners of the cavern, was a grown man's skeleton, complete to the last tooth.

King had brought a compass with him, and if that was reasonably accurate then the arms and legs of the skeleton were exactly oriented, north, south, east and west; there was an apparent inaccuracy of a little less than five degrees, which was no doubt attributable to the pocket instrument.
One of the committee members tried to pick a bone up, and it fell to pieces in his fingers.

Another man touched a rib, and that broke brittlely.