Think about harmonious color combinations - it looks like you just added red on the knight's armor because you thought it would look cool, but it kind of clashes with the green of the grass. Also, I like how you've created the illusion of depth by creating the darkening shades of grass, but this ruins the focus of the piece. Where do you want the player to look?
The place with the highest contrast in the piece is the hilt of the sword, dark against the light of the sky. The place that grabs the secondmost attention is the darkest area of grass. Wouldn't you rather the most eye-catching place be the knight fighting the monster? There are many ways of doing this, such as increasing the contrast between that and its' background. Try reducing the number of colors you're using and find a harmonious color scheme as well, it would unify the piece and make it more appealing. Make the layers of grass less attention-grabbing, reduce their saturation, and try to make lines converge to the center of the piece to make it clear that the knight should be the focal point. The piece certainly has potential, I look forward to seeing your update!
