Hey there! :3

So firstly, your terrain. I like it - you have a good diversity with levels and palette - the idea of sand and gravel hasn't been seen used like this, not by me anyway! I really like it, interesting concept. I like the floating island, however I feel like the land underneath it could be used for something (I'll get to that ;D)
I think it'd also be a nice idea to detail the terrain with grass/flowers just so it doesn't look so simple. The sand area you can decorate with a few dead bushes, or maybe even build your own cacti! (That'd be another organic)

Secondly, organics. You have none - this is where the part of land could be used! It's a really nice place for an organic, specifically a tree! If you struggle with trees, I recommend searching 'Plutouthere' on YouTube and checking out his tutorials. He's really well known in the community for amazing trees, and his tutorials are super simple and great to learn from :D

Lastly, your structure. Instantly, I was drawn to the palette. It's interesting, and I think it works, however the detailing is throwing it off slightly. There's a few too many chisselled stone blocks; it makes the tower look a little too overdetailed. You could have normal stonebrick blocks between them, work out a pattern of sorts where it works, just because using them on all sides isn't working with the plot :p
I think the cobble walls and fences work as detailing, but again not on ever level - you can find some other ways of detailing; the use of slabs/stairs is the most common. But no slab spam please! They're nice to make shapes with; it's a great way to show creativity as well!
I think the smaller building is good, but because you only have one layer of detail, it looks a little unusual. As well, because the blocks you've used 'close in on eachother' instead of point outwards, it looks too squashed. (By this I mean the ends of the stairs touch with the other block, they don't point outwards - I can't really explain it nevermind XD)
Also, I love the propeller under the bridge! It's a really nice design and fits weirdly well with the rest of the plot.

Good luck!
- Dutty :3