Downwell is a very simple game both in its presentation and gameplay. At least on a superficial level.
Beating this roguelike on Hard mode was quite challenging, and required the understanding of synergies between the different item you can collect during your run, as well as understanding how to approach each of the levels, how to effectively use combos, when to break them, or how to best spend your gems.

Definitely no easy feat, but a fun one to accomplish.