I've been working on this environment slowly for quite a few years, and finally put on the finishing touches this May. It was a self-directed project for the honors program at my university. My main inspirition for the style of this project - and honestly my inspiration for most art I make - is the game Poptropica. My main goal for the environment was to make an immersive and beautiful 2D space and practice my programming. To make the area immersive, I layered the trees on the z axis to have some parallax, and put low-opacity planes between the layers to give the effect of atmospheric perspective- that yellow haze. I also programmed in some shadows so the trees create long shadows which change in perspective with the player, giving the effect that the sun is right there in front of you. I practiced my programming through implementing those shadows, perfecting a pre-existing character controller to have a weighty jump and balanced speed, and making sure the platforms can be jumped on from direclty below.
Here's a player standin to demonstrate the platforming on the willow tree.