To find the tunnel to reach the Spirit tower, go to the 2nd level of the castle and go through the left door at the back of the castle to reach the guard who's blocking the way. You must have the sword to continue through.
To get the sword enter the castle and go to the door through the hallway on the right. Talk to the guard captain and he'll give a short tutorial on how to use the sword.
If you're still at a lost, here are some sites to check out. Link Link
ive alredy gone through the door and just helped the guard tht was getting attacked and he sed tht there was a path leading to the tunnel but a rock fall has closed it off and he doesnt know where it is and tht i should look for clues but i have no idea where 2 look