Started off super fun, then it started freezing mid-game. Plus, even with the upgrade tree maxed out the final boss seems unbeatable. I spent 8 hours replaying levels and gaining "rum" to try and make it more beatable, but to no avail.
Suggestion: Make the upgrades stay with the player so that they can upgrade the towers in previous levels. There's no need to keep level 2 difficult when you're preparing for the final boss. The last two-thirds of the game time was spent pretty much running in place.