Castle in the Sky by Zimtower is a short, sweet and beautiful music video. It's somewhat referencing Dalaran as a castle in the sky, but is really more focused on the idea of a "happy place" where each person can revisit their own childhood. And yes, that's totally a reappearance from the fun gnome from Cloud 9.
It's a pretty short video, so I wouldn't recommend trying to read too much into it. However, Zimtower has done an amazing job animating his gnomes so that they actually line up against the song's lyrics. The video is beautiful, with a whimsical color palette that serves to magnificently reinforce the song's message.
Ultimately, this is a great video. I had a fun time watching it, and expect it to be one of my go-to "pick me up" films in the future.