when you graduate from Apprentice to Master, you have two options. Either inherit your old master's world and reshape it to your liking, or just go off and find your own. When Aqua finally accepted her role as Master, she went back to the Land of Departure and reshaped it into Castle Oblivion. A place to hide and honor Ven's body, for one. And a testament to her commitment to the memory of her friends.