I guess there is a chance that it is another entrance to the Palace Amusement tunnels, it would be beyond belief that tunnels that size would only have one entrance, but I didn't think they headed in that direction.
But it may simply be something to do with utilities, if it had just been another isolated tunnel it would have been capped instead of rehashing the building that was over the length of that wall.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates