Downwell is an tight vertical shooter in which no single part of the game is wasted. I've tried many times to explain exactly how smart its design is, as has Adam, but it can be hard to appreciate the beauty of a clockwork machine by someone describing cogs and pendulums to you. This recent video from GDC 2016 might help: its Downwell designer Ojiro Fumoto explaining how he designed the game around its "one key mechanic," the gunboots.
Watch it here:
He goes through the game's design process build by build, from the initial spark of inspiration ('Spelunky but on your phone') to adding the gunboots for the first time to realising that the gunboots could be used to fight enemies, to aid movement, and could link into a combo system and... well, everything else in the game. The whole game is designed around maximising the player's use of the gunboots, and the video covers things such as how the ammo reload system was tweaked to support that.
Spelunky and Downwell are important games, I think, not just because they're excellent fun to play, but because their designs are clear and simple in a way that makes design seem accessible. Spelunky inspired Fumoto to make Downwell and Downwell seems just as likely to inspire someone else to make their own new thing. The video will help, too, in that it depicts the creative process where so many other things fail to do so.