The upgrade costs are a bit off. Seems like the only one worth putting skulls into are the Zombie Horde. It quickly become more benefitial to give in 1 extra upgrade than bying the perk for the Giant Zombie. Even buying the Giant Zombie in the first place, seems not to be worth the skulls. Nice gameidea and music though!
 "Zombidle"){kind=icon}
 "More Bloons"){kind=icon}
 "Max Damage 2"){kind=icon}