Sorry for the length. I'm not good at level design yet, so this is the best I could offer right now. Also I kinda ran out of time before I could think of any idea to expand the level. That's why I used the remaining time to polish the current level instead and that's the result.
It'd be nice if you could point me out to learn more about level design. It's rather hard to find a good one when the internet is saturated with a bunch of contents like this. I mean I tried to find something, but they're mostly just generic how-to which I kinda knew them.
Well maybe I still don't really understand the philosophy behind it, but I do know the how-to stuffs since I researched them extensively.