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
thief
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.
hahahahaha! A thief! There is no use eyeballing me with your two eyes, thief. I have nothing  What would I want with a spiders belonging? Silly.  You do not know what I have! Look at this jewel, bright and shiny .. wait! You did not see that! You stole my jewel! GIVE IT BACK bwhahahaha! Make me! You with your  two legs.. you are no match for me! I could step on you in a heart beat spider. Now look, we should work together here. And what would be in it for me?  What else do you have to do? Just sit here in this hallway and wait for the bugs that aren't going to come? Lies! I am the best bug catcher in the whole realm! Bug tremble before me! I didn't know spiders could do stand up comedy! Well we've plenty of legs for it  And plenty of eyes, now keep them peeled for me! I am going into the kings chambers.