Thank you for your feedback, it is really appreciated! I have uploaded a new version, trying to take some stuff you and others have said into account.
Larger critters now have additional explosion effects.
The cannons whine and burn a little whenever they get hit.
Changed the health bars so they're less distracting and easier to see how much health you have left.
Controls are explained in the description on Kongregate, but I've also included a link (question mark) in the game to a documentation webpage. I'm still working on that.
The animated background has become brighter, but can be switched off with a toggle button in the menu.
The game will have music, which should give it more "weight", but it's not done yet.
I'm still working on it. More feedback would be very nice! By now I'm so adapt at playing my own game that it's getting hard to judge its difficulty.