Level Design Philosophy

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Breathing Room - occurs frequently between challenges in Mario games. I recommend Ceave Gaming's video on Mario level design seen Here for how it works

I wanted to single out this part in your post in particular because breathing room is so underrated. I've seen so many games, even commercial ones, that are just non-stop action. It's super overwhelming if you get nothing but enemies around every corner, it gets really tiring and boring quickly. You need to have both highs and lows.

That goes for every piece of media, really, now that I think about it.