@Mozzah, at the bottom left of the defence screen there's a small icon that says "Buy defender" or something like that for 15 gold, click that when you can afford it to place one :) Enemies give you more gold as difficulty increases, at rank 1 you'll get 2gp ea and at rank 2 it's 3gp ea etc etc, hope that helps :)
 "Dude Quest"){kind=icon}
 "Arkandian Crusade"){kind=icon}