The level you didn't finish is l3300n.
The red key is more to the left, after you get the yellow key and it's near a robo sheep.
If my art is getting better, it's because I got a bit more aware of making colors less "eye hurting" and tried making some faces less weird in some stuff.
But the lack of shading and weird resolution is still there.
I'm aware my game will always be unpopular, but I wanna make sure it's finished and that I get to make more games.