I thought about it a bit more, and it's probably not that the controls themselves are hard, but because there is no background and the boxes shadow is so opaque that it's kind of hard to see how you're moving the outer box. I feel a textures background might make this game a lot easier to control (maybe it would be nausea inducing though).
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Yeah I actually agree with you here and it's something I considered during development. I too was worried about the nausea aspect which stopped me from adding it. But I'm thinking now that maybe something like a really faint grid would be ok. Something that you can consciously ignore but would sub-consciously give context to the level's movements.