Very nice, a couple suggestions in the body
Very nicely done. I've always been frustrated by the "this is the path we are taking" mentality of many TD games. A few ideas:
1) In the "pop-up" for when you're about to upgrade, show what the new stats for the cannon will be, so you know what you're getting into.
2) Make the ants a little more single-minded (i.e. once they get a piece of pie, have them head back to the hill on a more direct path). This will also make the game harder, so you may want to adjust the HP levels accordingly.
3) Have difficulty settings. This can range from the speed of the ants to their HP levels to their intelligence in trying to avoid cannon setups.
Otherwise, very nicely done!