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
king
I am a king of the whole empire. I give rules and pursuit them. I am brave and fearless.
Thief!  I have caught you trying to enter my throne room!  Explain yourself before you are thrown to the dogs! I I I am n Not a theif. I was just going to take this and have it washed for you The Royal Coat?  Unhand this at once!   Do you know who I am??   There are rules in this kingdom for thieves!   If you would just show me the way to the wash room I will have your maids clean these things at once! And it is a shame your room is in such disorder. I will have them flogged, Yes? Very nice try!  I know who you are.  Daniel the troubled.  If it weren't for your father and the fact that you have caught me sneaking out to see my mistress than you should also be chained in the dungeon as he was.   How did you find these secret chambers? I am a thief, how could I not know about these tunnels. Here. Thanks for not being too angry with me. My dear man, aren't we all thieves in some way or another.  I need you to do a job for me, are you up to it?  Or shall I ready your shackles in the dungeon? Am I!? I'm the best thief in town! I was born ready! Yes, yes, your reputation does precede you.  You may know my rival, King Anchilles of the West Realm.  He has a hidden room that holds a magical stone.  I need that stone!   It has been the reason for his great success and I fear he will never be defeated unless we have control of the stone. Ooh, sound like this is going to be the best heist I have ever performed. This will be written in the history books my King. I will do it! As you know I rule this whole empire, so I will have my men report back on your actions.  Do not fail me thief, or you will be thrown into the dungeon! If I fail you will know I am dead. Don't worry about me. I will get it done or die trying.