Provides a chance for an upgrade in Defense Workshop to level up for free after each wave.
Each Free Upgrades workshop (Attack, Defense, Utility) consists of 99 levels that increase by 0.5% per level for a total of 49.5%. The Free Upgrades workshop enhancement applies an additional multiplier to your Free Upgrade chances.
https://the-tower-idle-tower-defense.fandom.com/wiki/Free_Upgrades#Free_Defense_Upgrades