this secret tunnel is like a vein to the castle.  It leads to many other rooms within the castle.  This includes passage to the dungeon, the throne room, the kings chambers, and a secret exit to the outside.  You must be careful because the tunnel is small to maintain its secrecy.  There are 5 different rooms to the castle that can be accessed through this room. This also proves as a good escape route as enemies with large armor will not be able to fit through.
Secret Tunnel, Inside Castle
king
spider
I am a spider. I am large, black, and have eight hairy legs. I wait to trap insects in my web. I enjoy eating unwary prey. I live in a darkened wood near a remote village.
Hello what are you doing in my tunnel. Your tunnel? I dug this tunnel myself, you have no right to it! I live here I catch my food over there. I am here all day and night. You are never here this is my tunnel. Just because you make use of it without my say doesn't give you ownership, damned arachnid!  Well whatever you say. Is there something i can help you with. Well you can cut that out and leave here before I force you out! So you will live down here just to make sure I dont come back. Ha that is funny.  I shall not live here, I shall just make you leave once and for all!  I know of all the nooks and crannies of this place. I could live here the rest of my day without you even knowing I was here. Not if I force you out first, same with that damn thief that is with you! King, please there is no need to be violent. I just want to live in peace. Where are me and my children supposed to go. If you are to live in here, you are to know I still own it and that you should be grateful!