Here is the spritesheet so far for a game I've been working on

The axe, stone textures, and fire balls are all edited mario sprites (I'll try to change that at some point), the rest is my own or my friend's.

I'm looking for advice on how to improve the castle, and the backdrops. Other suggestions are also appreciated. Thanks!

I observed and analyzed your example image and I noticed some aspects of it that I would change.

Then I went through my bookmarks looking for pixel-art artwork to use as reference. I enjoy the work of Henk Nieborg (pixinspace), so I chose that one.
The reason that I went to look for reference artwork is that I needed to fine-tune my early perceptions of your work based on industry-proven, professional pixel-art. Get a taste of reality.

.....

After observing and analyzing Henk's work, I realized that he paints everything to look natural.
In his work, a ground tile is not a perfect square block. Henk conveys an irregular volume; it looks like an actual piece of rock or dirt.
The overall impression is that all of the elements (the rocks, trees etc.) have natural imperfections, adding realism and depth to the scene and making it look like an illustration.

This is what research is all about.
Armed with this analysis, I reflected again on your example picture and came up with the following recommendations:

1) I will talk about the tree sprite since it has some important points to address.
Some of the pixels on this sprite seem double-sized and others are not, as if someone painted most of the graphic with a 2x2 brush instead of a 1-pixel brush. It breaks the illusion since we can tell individual pixels apart.
There's an ambiguous lighting on this sprite when you compare the foliage\crown part with the trunk part. Is the lighting coming from the top, or from the front?
Additionally, you could be much more economic with your colours. The shape is almost the same, I just recoloured it to illustrate these points.

2) To make the castle more appealing, rotate it to a 3/4 profile view.
The following isn't how I expect it to look when finished, it is just a rough draft of how it could look more interesting simply by rotating it.
Additionally, based on that research, I would add a lot more imperfections to make the castle more organic. On your castle, every single brick is the same. Why not add variation and make the bricks only appear on the corners and where the wall is broken, as seen in this other piece by Henk (shantae06.gif) ?

3) The backdrops look a bit flat and too saturated.
For them to actually stay in the background you need to slightly desaturate them so that they don't fight against the foreground objects for the viewer's attention.
For them to convey depth you need to add hazing to the farthest elements for atmospheric perspective. This means that the farther that an element is on the backdrop, the more its colour is blended with that of the sky.
Even with these corrections on the backdrops you still need to do a mockup of a scene from the game, which is nothing more than arranging all of the sprites, tiles and backdrops to look like a screenshot from the game, with everything in place. This will let you see if there are any problems with harmony and focus.

@Kryzon Thanks so much for your detailed reply! I was aiming however for a SNES style of 8-bit art, and the screen size I am working with is only 318x218. The example backgrounds you provided are way too detailed for my liking. I love what you did with the tree however, and I will take your advice with my backdrops. The castle is interesting, and I will probably use something like it. Thanks again for your time!

Here is a mockup, note that the background more washed out than in the original post:

Have you ever thought about writing up an article with some generalised tips on properly researching and designing an art style?

That's a good idea Jason, I'll keep it in mind.

Here is a mockup, note that the background more washed out than in the original post:

To me it looks improved that way. It feels much more deep.

That mountain range backdrop with the purple sky has those separate layers that you can play with some atmospheric perspective.



If you were doing a more modern visual style you could also blur the deeper layers to add a "cinematic depth-of-field" quality to it. The effect looks like this, with deeper objects slightly out of focus (though you can also do this with extremely close objects). The game Limbo uses this effect to a great extent. But I digress.

@Kryzon, Thanks again for your help. I think with the more faded backgrounds, and the new castle I'm working on, the game is looking much nicer!

Just for future reference, another good place to ask for feedback is the "WIP" area of PixelJoint.
http://www.pixeljoint.com/forum/forum_topics.asp?FID=8