When your in the castle at some point you will be in the circular room and there will be these square pieces all over the place. Use each of your characters to carry one then place then go down the stairs in order on the lower level. They will form a picture. That's how you will open the gate to proceed to the further down the level.
When you come across the white witch. A ware wolf and a dwarf will keep coming after you. After you've hit the ware wolf a few times he will go back to where you cant hit him. This is where you should get near the side for safety. He will unleash a powerful howl that will shake the ceiling and knock down some boulders from the roof. Pick up these boulders (This is much easier in two player and quicker) and lock onto the creature chanting and throw the bolder at it. Do this a few times and you shall save Caspian and take out the White Witch once and for all.
there the white thing by the chest keep touching it , and it will roll and go walk over it ! I swear its the way to get it .