Challenging but fun
It didn't hold my attention for long though... because of the keyboard controls.
It would be a lot more inviting to retry after failures if you didn't have to use 3 different keys for level navigation.
Instead of Ctrl, Space, and Enter being used to start/retry/advance just one key (preferably near the controls) would be advantageous. Like have ctrl do all three or something. Also, some of us have gotten used to ASWD controls for keyboard stuff since the right hand (for most people) is dominantly on the mouse. Adding ASWD as alternate controls with possibly the space bar being the start/retry/advance key would be a nice addition as well.
Other than that, I didnt get too far in the game but I didn't see any real problems with it either.