Puzzles started easy and gradually became pretty challenging but nothing too mind boggling.
For the controls, there are two implementations I could suggest to make it better, you can make it continuous (I see many have suggested this ) or make the move animation faster/snappier. I understand you were trying to move the characters per grid which is nice for such puzzle.
Other than that, enjoyed it a lot. The last puzzles are great!
P.S. My game also has robots :)