At the first part of the game, you enter the palace to find Tetra. In the room Tetra is in, she is locked behind bars. To get the key, you must defeat a stone-knight. If you go up close to him he will hit you. Hit him with your boomerang. While he is still in shock, hit him with your sword as much as possible. Do this until he disappears and leaves behind a master key which will open any door.