I love this concept. The floating islands are super rad and creative. I also like the general shapes of the city itself. As far as style goes, I wish there was even more pen detail. As it is, it feels like a pen drawing with a quick color overlay. I'd like it if the two styles were more cohesive and working together. The foreground also could use some more elements. It feels too empty currently. Maybe just crop it in a bit and get rid of that blank space all together.
I'm a total sucker for the whole floating castle thing, so obviously I love that :) There's a fun sense of depth there, but I think the path up to the castle is coming off a little flat, maybe wrap the path around the shape of the mountain? Also, I'd overlay some of the sky blue over the floating castles in the background, to add some atmosphere :)
The idea is really neat and the silhouette of the city is interesting. I kind of wish it was a wide shot so we could get a better perspective of how big it is, especially since we're seeing 2/3 of the city...seems kind of small.And of course, cleaning up.