Music was...
having that kind of music all the time was a bit annoying. Good game though. I thought the difficulty was about right but I'd like to know the high scores (is there a high-score function?)
What worked well for me was upgrading one cannon for the most part and having the other one weaker. Of course, it got a little hard when enemies came to the other cannon's side, but just by accounting for the distance between the main cannon and the enemy, I could hit them allright and usually with both shells