Hello!
This time, I made the following updates.

• I have taken emergency action against a bug that causes the game to become unprogressive on level 15. As far as I've checked it seems to be ok, but please let me know if you still have problems.
• Implemented options to invert the viewpoint control. More precisely, you can invert the viewpoint control by setting a negative mouse sensitivity value.
• Minor game balance adjustments have been made to Challenge Mode.

Please continue to send me any problems or requests via the 'Report bug' button.