Why can't I get in to fight him? Just a door that won"t open. I believe I have completed all the quests up to this point. What am I missing?
When you look at your map in that area, you will see three rooms / areas on the outer edge of the map. There are three other bosses in those rooms that you must fight first.
When you enter the area, stay along the right side of the wall, and it will lead you to the first room. As you defeat each one, and leave their room, continue to stay to the right side of the wall, and it will lead you to the next room.
After the third one is done, then you meet Prince Aka.