Hey there :3

I got your mail.. So I'm going to try and write you a long and detailed review, so you can really start changing some things up!

TERRAIN

So first of all, I'm going to talk a bit about your terrain. Ideally, we don't like seeing flat land on a plot. Basically, this is only because it obviously doesn't show much creativity in the way of that aspect of building. Terrain is underlooked, but there are actually people out there who only build terrain in Minecraft, or build terrain mostly anyway. It's actually quite complex but people tend to ignore it. I think terrain is a huge importance to any plot, because I like looking out for atmosphere. Atmosphere is created when you stand on a plot and it feels realistic, almost. When you can walk around and things aren't just placed randomly - they have purpose. And that's what I like to try and convey through my builds, or like to get others to convey through theirs.

Atmosphere can be created through various ways, but in terms of terrain, I think it's widely made through cliffs and layers to the grass. This can be something simple like this:
http://www.reverieinteractive.com/BetaGames/Minecraft_Clone/MCCloneTech1.PNG
This is a little more complex than what I was aiming for, but it shows you the layers well. Some can be jagged, some can be smooth. You can make as many as those amounts of layers, or you can only make like 3 or 4, it's completely up to you. That image is justa  good demonstration of how you could make natural looking flatter terrain to build structures on! This is kind of what I wanted you to show in your terrain - more layers than you have currently.

You can include some water as well if you would like. It can be nice, and atmospheric, to have rivers, waterfalls, small ponds etc. Just adds to the environment and can be really pretty. Of course, when you do this, you want to detail that area too. What I've seen a lot of people do is use lily pads, sugarcane, wheat etc to decorate around the water.
Examples: https://imgur.com/a/U0Eq0#0
(Credit to squarishbinkie and _Ems_)
These are a couple plots that demonstrate this well. It's just nice to have that little bit of colour around the water.

Another nice idea, which I know a lot of people use to add atmosphere to terrain, is adding wheat and farms.
Example: https://imgur.com/sUMbRpc
(Credit to phantomblock213 - Spark plot)
This basically is a an easy way to fill space and make everything feel more realistic and pretty around. Sometimes you can make them this detailed, or you can just make a block shape of wheat and fence it off with cobble walls and fences.
Example: http://i.imgur.com/q45VZZZ.png
(Credit to DuttYGilberT)
This is just a nice way to fill space, adding hay bales in so its not all one thing, and the occasional spot of water - one to keep the wheat hydrated, and two to look nice ;))


ORGANICS

For your trees, I recommend you look up this guy - Plutouthere - https://www.youtube.com/channel/UCLUKySjsOfU9jcjYyFJWk2A/videos - there’s a link to his YouTube, where he has a couple tutorials on creating different kinds of trees. Check that out and see what you think. The larger tree tutorials might help you with the roots/branches. The smaller tree tutorials may help with adding to the atmosphere.

For the dragon, here's some pictures of a dragon made by a person on the server called rosalinda316. It shows the curves, the shaping and the proportions of a dragon - the wing also shows what I meant by using a different block type instead of all black wool:
https://imgur.com/a/0kVhL

Try and use inspiration from those images to help improve your dragons.


STRUCTURE

Your structure is very good. I like the design and the shape that you have to it, and a nice palette. But it needs more detailing and depth.
This part of the building especially:
https://imgur.com/8h8pcmz
You could add some stairs to create more of a shape. It'll curve the areas where the shape curves (the edges of the circle).
You can also use full blocks, like andesite, to make a shape.

This for example is something I've been working on.
https://imgur.com/5GQ3lKY
Notice the detailing on the walls, with multiple layers so the wall isn't completely flat. The area with the redstone lamps could be where you have your windows, for example.
Adding stairs/slabs can create curves, so the building doesn't look so blocky. This can also work on the shape of the castle you have, just don't put the detailing on the walls of the towers, like on the front - this'll stick out too much and make the detailing look too 'placed' and not thought about.
For example:
This kind of thing is okay:
https://imgur.com/H5l1WVQ
This isn't:
https://imgur.com/XKHOf2t

Because of the castle theme, it might be nice to have some stalls around the castle, on either side maybe - this'll fill some space as well, and add atmopshere! :D

I hope this can really help!

Good luck!
- Dutty :3