Since you humbly admit to not being an artists, your initial priority should be in studying references.
Find all the pictures of castles that you can and study their natural construction, then find all the pixel artwork of castles that you can, study how that artists composed their work, decide what you think worked and what didn't, try to make some practice pieces mimicking their techniques.
You're pixel art is fine if that's the style you are going for, but based on your post I think you want to refine your skills and add more detail and depth to your art. At this point, start considering yourself an artist, and start working the way an artist would, by practicing and using references and re-working your previous pieces.
Really take objective looks at your work and think about what doesn't look right and why, so lets take a quick look:
Picture 1 - Palette, your right on, the palette is a bit monotonous and the picture is blending together except for the bright white guy in the middle. Use some brighter browns, closer to yellows for the table and door, there's a copule windows in the picture, maybe they create some light rays in the room? Consider where you want the light source to be outside and inside, speaking of which, you've got a fire up on that wall, consider the radial glow from this fire also! Maybe the floor of the castle needs a rug or some different color stone?
Picture 2 - again consider your light source, the rocks around there seem to all have their own individual light source, remember that the shape and structure of objects like rocks can block light from hitting lots of other parts of the rock. And other thing to consider is proportions, you're using a rather small scale for your pixelart so take a look at other small scale sprites, they manage proportions of the character and environment differently, some details are omitted while others are exaggerated since you only have a single pixel width for things like the arms and legs of the character or props.