I kind of disagree with CelioHogane in regards to limitations. Using limited colours for pixel art is usually a way to make it easier, not harder, in my experience. It's kind of like cooking. If you're making a meal with only 4 different ingredients, then it's easier to get a good balance and increase or decrease as needed. If you're making a meal with 100 different ingredients, then it's a lot harder to get the same balance. If you suddenly add too much flour, then you need to adjust the 99 other ingredients to make up for that.
Of course, it's hard to make a pixel art masterpiece with only 2 or 3 colours. Just like it's hard to make a four star Michelin dinner with only 2 or 3 ingredients. So there's a balancing act involved too.
In regards to the first background, I think you need to do more work on the trees before we can help you with them. Start refining their structure, the trunks and the leaves and the vines. Then we can provide feedback. Right now it's too rough to help.
In regards to the second background, I have two tips:
Use a lot of reference images for the holes in the wall. Google stuff like "rubble", "ruins", "wall hole" and so on. Don't draw it from scratch with nothing to inspire you. Try to imitate something.
Don't add texture without a clear idea of what kind of texture you're making. A lot of people will just add random scribbles to make the texture more complex. You've used some dark lines. But what are they meant to represent? Are they cuts from lasers? Are they semi-broken glass or ceramics? Are they splashes of dirt? When you make a mind up, use google images to find a reference and try to imitate. If they're cuts or cracks from lasers, then you need to add both light and shadow to make them look more 3D. They will usually be lighter on the bottom 'lip'.