Very fun game and novel idea! Enjoyed every second of it. A few critiques:
- There seems to be some scaling issues with UI (my screen resolution is 1920x1200.)
- More variations of colors could be nice. Someone else already suggested to put given numbers and numbers you fill in as different colors, and maybe you can put the axes as different colors too, or something
- I know this game is aimed more at advanced players, but as someone who doesn't have a solid background in normal 2D Sudoku, even the first puzzle felt too mind-bending >_< wouldn't mind if you included an even simpler introductory puzzle
- If you accidentally mistyped a number, it's hell to debug at the end, scanning through every layer to find the error... could you add a little hint to help with debugging, maybe let the outline indicate whether the current layer has an error or not, instead of whether the whole puzzle has an error or not... (maybe it can be a toggle)
- Is there a reason you can put a 0 in it?
- The pause and resume icons should be swapped. When the timer is ticking, the "pause" icon should be shown, like YouTube video player
- When you pause the game, the music should be paused too
- It's unclear that the X button clears everything, better to just say "clear"
- Save/Load doesn't have a confirmation prompt, so I'm afraid to click them.
Again, this is really well done for a first game, kudos to you!