Very cool concept! And nice challenging levels. The UI could use some improvements. Some suggestions:
- a reset button to redo the level (when you see it's not going as planned). It's annoying to wait until the end of the game and then select the level again
- using the scroll wheel to change the time is not bad as a shortcut, but a separate time control for each piece would be more intuitive. It is a bit confusing to me that the clock shows the time for the piece you want to set and also the timer for the game.
- the king can't move, which makes sense with these puzzles. Maybe give him a different color?