You have to get the crushing stone first then use it to break all the cracked walls in both castles. One wall would lead you top Juste's friend and you will fight him. After defeating him, he will give you the MK bracelet. Go to the sector where the golden door is located wearing the MK bracelet and you can enter it. Some doors would need the Lure key and the Skull key so you have to go find it.

Could you help me out with this one? I can't find it.

you will meet death and he will escape the portal that leads to castle b.. Crushing stone can be found in castle b..

how to open the door of speel