If you have placed a Tier 4 tower for more than 20 rounds and you also have a Tier 3 Specialty Building for it, you gain the abillity to get it's elite upgrade at a cost of money. Elite upgrades can enhance the towers attacks and abillities and can even add new attacks and abillities. If you use an Elite Training Dummy, you can upgrade an elite upgrade without having to wait the 20 Rounds. Here is a list of each tower's Elite upgrades: