Talk:Corrode-O-Matic/@comment-26546865-20150708033612/@comment-24170389-20150708213640

They're premium upgrades, which improve your towers in some way for 1 game [or unlimited ones once you bought with NK Coins].

Example: If you buy "Double Rangas" premium upgrade, your Boomerang Throwers will shoot 2 boomerangs instead of 1. (That's available on BTD 5, along many other upgrades, if you didn't notice)