I also like the sound a lot, see my comment below. You might want to check out my Game, I created music and all sounds myself and it got well received so far (someone wants my voice for his daily routines :-))
Anyway, let’s focus on your game. I like the visuals, very sketchy programmer art, but super funny to look at. And, as said, very fitting sound effect. I also love the additional details like the pig.
From the overall idea, it is not really an opposite Tower Defence, as it still kind of works like Tower Defence. Maybe force the players to put cannons and let them only proceed if they can put them in positions that the enemies get through.
A minor fix would be to make the level restartable or a rebalancing, as I often had the case that I put the cannons in a way that no side would get any points and it runs forever. Then only a reload helps and you have to start from the beginning.
All in all: solid and certainly a good basis for further explorations.