If you accumulate more than 2.1 billion rocks (2^31) while the game is closed, when you re-open the game, the number of rocks will overflow, become negative, and you will lose money if you sell your rocks. It seriously hampers "idle" play if you have to open the game every 8 hours to sell your rocks. Can you make the number of rocks a 64-bit quantity, or maybe a floating point number?
There's a serious bug in this game. 10 seconds after I started playing, the mouse locked up. It stayed like that for a while, so I tried Ctrl-Alt-Del to kill Firefox. My screen went black, no task manager, but I could still hear the game music. I eventually had to do a hard reset of Windows. When I came back, I logged back in to Kongregate. My level 58 account was still there with all my badges, but my local Kongregate storage was empty - all my progress in every game was lost. Lesson: back up your Kongregate saved games, and don't play this game.
How do you load a game? I played to level 4, and got "Game Saved" messages, but when I closed my browser and re-opened, I only see the option "Click to start a new game", which puts me back at level 1.
BTD5 has many nice features, but one flaw makes it worse that the other 4 editions: you have to manually collect bananas from your banana farms. Sorry, but that's not how a TD game is supposed to work. You build the towers, and the towers do the work.