The architect responsible for this should be jailed in it for life.

You don't have the correct roof angle, but I must say it's an improvement on the design.

City lights reflections are never angled as you made them: always vertical, maybe fanning out at the bottom. Check your night view ref.

The dark green (?) is of almost equal value to the dark blue, doesn't stand out and shows almost lighter (on my monitor).

The sky is too noisy. I don't know if you tried to make it as in the ref, but I think that sky has too much detail for pixels. I'd suggest to make it more calm. The saturation gradient looks wrong too: might be better more sat on the horizon, less so above. Tough palette for a dusk sky anyway.

I'm very new to pixel art and unfortunately cannot draw at all so I have to pixel it down straight away.

Nonono.
Drawing and pixelling share the same basics. Practising one is helpful to the other (both ways).
I strongly suggest to start sketching, pencil on paper. Start simple, it's mostly a question of learning to look.
Sketching is more natural and much faster than pixelling, it allows you to try more stuff in a short while.
Just do it everywhere and at all times, it's fun too!

I think it's too noisy, esp the grass.
Also the walls look paper-thin. Castle walls are supposed to be massive.
Needs some cast shadows too, both on the walls and grass (no idea how to do that with tiles).

Why ask for advice and upload to PJ without waiting for answers? Way of wasting pples time.

Wonderful ref. Also helpful for pointing problems.

Adding to ndchristie's point about perspective, look closely how the water edges get more linear and horizontal in the distance. That's a powerful way to convey perspective and depth, linked to the observer's point of view height (not much above water level). In this respect, the further bank in your pixel looks tilted.

About color use: you are using colors you think stuff has, not the actual colors which depend on light, point of view etc. In short, your trees are green, trunks and rocks are brown, water is blue. Not so in life, not so in the ref! There's actually very little green in those trees and grass, very little blue in that water.
But some colors (dark greyish/bluish purples) are reused all over: color conservation is not pixel-art-specific! This is a very strong way to unify a piece.

Distant mountains are more blue than the middle-ground cliff.

Then, shading. Light mostly comes from a low sun behind the trees on the right; it is both soft (no sharp cast shadows) and contrasted (foreground doesn't get much light).
I'd suggest to straighten this before any other consideration, because the dark and light patches have a very strong influence on the overall composition.

Another composition point: look how every part of the painting draw the viewer's eye toward the central details (mountains, cliff castle and waterfalls). The tree on the left is important for this, whereas in your pixel nothing prevents the eye to wander off the image on that side.

You're seriously challenging yourself for a first try, people and cloth being the hardest things to draw. And in isometrics too. 

You should definitely start by practicing with a basic nude figure first, and add clothes later.
And you should definitely start by sketching people on paper, from both life and photos, in a lot of poses, and start pixelling afterwards.
The point is not (yet) to make good drawings, just to teach your eye and hand how people actually look.