When an invalid (red) piece is hovered, the pieces protecting it are now highlighted so you can see exactly what's wrong.
This is a feature I'm still uncertain about and would like more feedback.
Added a quit button so the game can now be controlled entirely with the mouse with no need to touch the keyboard to quit.
Fixed a visual bug when closing the game when inside the instructions menu.
Added a windowed mode. To switch between windowed and fullscreen mode, press Alt+Enter.