Hey, that's great work. You did impressively with so few colors, especially on the tree trunk. I have some suggestions:
• Add another color on the lighter ground. It's really hard to tell one layer of that ground from the other, so add a slight outline.
• The dirt looks really flat in the middle areas. Add some rocks or something to break up the monotony.
• Change the green. It's a weird neon color that doesn't fit well with the desaturated ground. Make it darker, etc.
• The area under the door/hole doesn't read well. I think it's supposed to be something you can walk on, but it looks like something you're supposed to jump through.
• I agree about adding extra stuff. Just make little bushes and vines and stuff. They don't have to be exciting on their own, in fact they should be subtle. But it is very boring without some variety when you're playing a whole game.
Like I said, good work. I wouldn't bother writing this if I didn't like it
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Edit: cool, I didn't see you had a game. I'll try to play it this weekend and let you know what I think about it.