A castle in the sky is a fantasy staple, a place of wonder and magic. In each of the WOW expansions there has been a castle in the sky, though they share little besides that general title. I love this song, btw.
This is Tempest Keep from the Burning Crusade. It is certainly a castle in the sky with multiple sections floating up in the air over the bizarre and otherworldly Netherstorm. It was not only the site of many, many wipes to Kael'Thas but also 4 gigantic crystal palaces.
The level of detail on Dalaran in Wrath of the Lich King is much better. It is a friendly, happy place, familiar and entirely human instead of a crystal fortress constructed by a inscrutable race of godlike entities. It is also ripped out of the ground wholesale and flown around instead of being placed here in some deliberate fashion. The feel really works, it is a fantastical human city torn from the ground by great magic, and yet still very much a human city.
This is the crowning achievement. The Vortex Pinnacle is in Cataclysm and it is the best castle in the sky yet. It is an ivory tower, a masterpiece of architecture floating on a cloud. Literally the buildings go down into the cloud you can see there and vanish in the mist. The effect is stunning visually and is a good showcase of the leaps forward the game has made in making beautiful scenery for the players to gawk at. Surely after I run through the Vortex Pinnacle a dozen times I will ignore the scenery and the gorgeous artwork that makes it so compelling but as I am still at that delicate, impressible stage I will gush a little and let the praise fall easily from my lips.
While I am obsessed with numbers and with the balance of the game I do find it so enjoyable to run around the world and see all these strange and beautiful things. There is just so much of me that loves the idea of exploring a fantastical, magical kingdom and seeing all the wonders and horrors it has to offer that for a little while my math brain can take a back seat.