Holy frick. you submitted a day before me, and i only have 2 levels. W H A T
Haha! I first drew all the levels on paper and then used unity to create all objects and saved them as prefabs. Once that is done, all I needed to do was to place those prefabs in each level. And when you play, you will realize that my game has levels that are really short too! That's why I had to make so many levels. Once you figure out how to solve it, you can finish each level in a snap!
But now that I'm making the last 15, I'm trying to make them harder.