Corrypt

Corrypt is an intricate and mysterious puzzle game. Explore a strange world, meet the weird creatures who live there, and solve the fiendish problems that trouble them.”it’s got one of those simple game mechanics that grows up through the levels to produce brain-curdling complexity as you go on” – RockPaperShotgun“this-is-impossible-oh-my-god-no-it-isn’t!” – IndieStatikThe PC version was named by indiegames.com as the #1 free puzzle game of 2012.

It takes about 4.7 hours to complete this game.
Difficulty of this game: Just Right-Tough