Alpha Mods are special modifiers that can be bought mid-game (and last the duration of the current game). The player needs to be rank 30 to unlock them, and to buy a mod to a tower, the player can simply double-click the tower and a small menu will appear with the alpha mods available for purchase in the right. The mods can stack (so 2 Ammo Mods increase maximum ammo by 50%) and the cost for the same mod is doubled with each purchase (1st Ammo Mod costs $2.5k, 2nd costs $5k, 3rd costs $10k), but each tower can only have 10 mods, and they cannot be sold. There are 8 types of Alpha Mods: