@Vaefio When you have a certain amount of minion ( 10, 50, 100, ... , 1k ), you will receive certain bonuses which you can discover by hovering your cursor over the circles.
When all of your minions classes ( skeleton - demon ) have unlocked the bonuses above, for example all minions have reached 10 or more units, the first circle will turn green, showing that you have unlocked even more bonuses
 "Forbidden Arms"){kind=icon}
 "Strikeforce Kitty 2"){kind=icon}
 "Overlod"){kind=icon}
 "Plunder Mars"){kind=icon}