Just for curiosity.

[Future Esthar]

This is Ulti´s castle.
Do you see that wind-like thing on the left of the picture?
It has an arch shape.But we do not know where it starts or where it ends.It´s like it was going around the castle so we can´t see it entirely.
What do you think it is?
I had been thought about this some days ago.
First I thought it was some time flow river.
But I don´t wanna to believe it.
Why did Ulti´s castle float?
Recently I realized why.
This wind thing is a ring which rotates around the Castle.
It´s nothing else than the Garden´s gauge.
In fact you can see that it rotates anticlockwise on the FMV.
The light we see runniing through the Garden´s gauge is seen on the Castle on the form of thunders (electricity).
Which means her Castle is build on a Centra shelter.

[ThroneofDravaris]

Well, I always thought that the idea of the castle was that it was a 'dark' parody of a garden. Just like Ultimecia's black wings could be considered a parody of Rinoa's white wings. So yeah, the wind might be some form of 'gauge' or whatever you want to call it.

I'll have to watch the FMV again though...

[Squall of SeeD]

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.

[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.

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 don´t know what it is,seriously).

[Xaven]

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 don´t know what it is,seriously).

That doesn't quite address what Squall of Seed mentioned.

[Future Esthar]

Thanks for the compliment Xaven.Maybe it´s really pyramidical.

Maybe the gauge is going berserk.
It also happens on the Trabia Crater.
However we see the entire gauge because the berserk was not as strong as Ulti´s Castle ones.
Time compression could had an effect on it.

There is a clock tower on the castle.The gauges there work.However the clock don´t seems to work.
These gauges were to control the Big gauge.

So time compression could have some relation with this big gauge and it´s misterious missing part.

[enigma1234]

Ultimecia's castle is in an another world(remember the portals?) and I think the dark thing in the left picture is a dark wind(remember when you walk in the halls were you will see the dark clouds moving? it only means that ultimecia's castle is in another world because you wont find her castle in the map and clouds only move's when a planet rotates it's axis, actually clouds doesnt moves but the planet does.

I think the reason why ultimecia's castle is floating because there is a dark matter below and above it(example: when you try to compel the north and south ends of the magnet and let go of the north side and hold the south end you will see that the north edge will float above the south end of the magnet) just like the castle of ultimecia.

Aren't you curious why there are chains all over ultimecia's castle?
Because it might get sucked-up by the dark matter(gravity) surrounding the castle.

[Skyblade]

It's the black wind!

The black wind begins to howl...

See, Ulti's castle is really the underwater palace. Ulti is trying to awaken Lavos.

Oops, wrong game.

[G SpOtZ]

i'm gonna bring my own theory into this.

when you go into the underwater research center, zell mentions that he heard it was for magic, with drawing and stuff...
but at the very bottom, you fight ultima weapon and you get eden from him. when they were researching down there, they discovered Eden.. and they started doing expirements with it, but then Ultima Weapon appeared, junctioned griever, and whiped out all the scientists. have you noticed that eden looks like balamb garden upside down kind of? and the biblical name is the Garden of Eden, yeah? GARDEN... yeahhhh... so the scientists that were down there a long time ago were actually the ones who came up with the idea for Gardens, but Edea took the idea and added SeeD's to it.

LOL XD

[GunbladeMaster]

HELLOOOOO!!!?? Ultimecia is a SORCERESS. She has magical powers. She can do anything she wants let alone make castles float. she accidenty overcasted float making it want to float away into Space. So she chained it down using her buddies that we have to fight in her castle. The Black wind thing is The Darkside Of the Force that is being controlled by Ulti. SUE SQUARE FOR USING STAR WARS!!!

[Future Esthar]

Don´t forget that magic has scientific explanation on FF8.

[CaladBolgLionHeart]

HELLOOOOO!!!?? Ultimecia is a SORCERESS. She has magical powers. She can do anything she wants let alone make castles float. she accidenty overcasted float making it want to float away into Space. So she chained it down using her buddies that we have to fight in her castle. The Black wind thing is The Darkside Of the Force that is being controlled by Ulti. SUE SQUARE FOR USING STAR WARS!!!

DUDE. GunbladeMaster totally just dropped mad knowledge on all your unsuspecting craniums, homie! AND he threw a joke in there?!? Two thumbs up. (Besides all that, he's probably right)

[GunbladeMaster]

thanks CaladBolgLionHeart

[muzzer]

my guess would be dark magic

[Future Esthar]

As I said,magic has scientific explanation on FF8.
Gauges,also.
So Ulti needs a Gauge to float the Castle (something that this magic would require).
Why Ulti wanna the Castle to float?
You know,you can think of magic as a substance.It needs to be stored,processed,etc...
It´s a substance(and/or energy form) whose mechanics were mysterious.But I think it is material anyway(not spiritual).It´s not that I am a skeptic.Is that it is how it works on FF8.
If Ulti can float just with magic why would she use wings for example?
Now imagine the Castle(an object with more weight than Ulti).
Magic serves as a fuel which makes the Clock gauges work and make the Castle float and move.
I will advance and say that there is time type magic applied on the gauge.
Now one can explain the mysterious missing part of the Gauge.
It was absorbed into time.
The entire castle is also to be absorbed into time.
During this state the Gauge is going berserk.