Cool!
I really liked the core mechanic. I would've liked more instructions. For example, at some point I soft-locked myself within grey walls with no obvious way to escape, and I was fully prepared to use CTRL+R to restart the game, but I discovered that R lets you turn back to the last checkpoint you activated and T lets you iterate over the checkpoints, from the first to the last activated one.
I loved the cube movements, the assets and the final credits. By the way, it was an amazing idea to place them in the 3D world like that. Kudos.