Quote Originally Posted by Future Esthar
And when one looks to the Castle one can see a resemblance with the conical shape of the Garden.The tower and the bridge also has a resemble.Nothing important at all.Just to add.
That is an excellent obsevation of how Ultimecia's castle resembles the gardens. However, I believe that pyramidical is a better word to describe garden's general shape but I suppose conical is okay.

Notice, though, Future, that the black mess of... whatever it is... goes off the bottom of the screen. It doesn't actually go around the castle itself like the Garden rings do.
Note how it has similar colors with the surroundings.
It has something that make it transparent(I donīt know what it is,seriously).
That doesn't quite address what Squall of Seed mentioned.