Thanks @cincipon, @kuso, your comments are very useful.
@cincipon, I've also had some concerns about the sound. But I've found it very difficult to find the right kind of music for free. When trying to remove a shape, you must slide it into the trash bin (at the top of the screen), hope that helps!
@kuso, I'll try to work out a way to make the easy level a bit easier (perhaps I could increase the opacity of the colours on the holes).
Thank you both again for your comments, I think they'll really help me to improve the game.