The Magic Kingdom is the remnants of the old empire of the same name.
When the flooding became severe, the King took the majority of the Magic Kingdom further North to find dry land. A smaller contingent stayed behind.
The Magic Kingdom is located around the park of the same name. The majority of its population stays in some of the Disney hotels or in a few cruise ships that surround it.
The capital of the Magic Kingdom is the Disney palace that has been raised to be above the water level.