Defences defend your base from enemy troops. Upgrade them to increase their damage output and hit points. Some defences like the Bunker and plasma gun only attack ground, therefore making it easy for a helicopter(often said as heli to shorten it in the game) to destroy it since it is a air troop. The battlecruiser is also a air troop, also making it so that the Plasma gun cannot attack it. There is a defence that only serves a barrier to ground troops. The wall. Other defences such as the tesla tower and the rockets/rocket launcher attack air and ground. They can target any type of troops. The anti aircraft tower is a defence that will only hurt helicopters, battlecruisers and protoplasm emitters.