My only issue so far is the absurd price of keys. Unless the coin drop rate is better at higher levels it's simply not practical. By the time I gain 5 lvls and clear many dungeons I can barely scrape up 5k coins and by then I've abandoned 10 chests because I can't store them all. Something needs to be fixed.
I personally don't think it's to hard on easy or a higher level of difficulty. That's the challenge...it's called a strategy game for a reason. I've figured the perfect strategy to get the game rollin' till you make enough cash to just spam towers and kick back without losing lives.
Reaching rank 31 without buying Kreds is fairly easy. It's quite an enjoyable game ,a great stride forward from the other Balloon defenses. Love the new upgrades and the rank system.
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 "Dungeon Rampage"){kind=icon}
 "D-Bug Defense"){kind=icon}
 "Bloons Tower Defense 4"){kind=icon}