Fun, but too easy
The game is fun, and has great visual and audio elements (love the song!), but is way too easy. The first problem is that the single box is not worth buying, since it's way cheaper to block with 2x2 sling traps since you don't actually have to rig them up to anything. The combo system is really cool, but the gold reward becomes so huge if you make a diagonal of flinging traps designed to just conveyor the pirates up the map that I ended up getting gold faster than I could spend it. After about three of these diagonals, I just started dumping cannon towers all over.
Also, at around level 49, my gold became NaN, so I had infinite gold. Is that a bug, or did I trigger a secret?
This game is a lot of fun, but I beat it on Hard mode with no leaks in less time than it took to do a load of laundry. Maybe in the next version you could make multiple maps with more complex terrain and with passable but unbuildable tiles so that the player would have to be more strategic about the use of the many cool towers that you made available.