Good game - and extremely difficult! Might be good to ease the player in a bit more gently with a couple of tutorial levels or easier ones. Good graphics. As others have noted, the music doesn't fit the game but is ok.
It is indeed difficult -- I can normally not get past level 7 or so myself! Thank you for the feedback -- I did think about having an easier on-ramp, but in the end decided it would just be hard. I think I'm the only person who thinks the music fits, which is also good feedback because it says to me "Stuart, don't trust your music choices, get more people to playtest first and ask them next time" :-)