I finally got around to adding Knights and Goblins. The Knight can be commanded, and Goblins will attempt to either destroy your builds or attack Knights, based on a random variable.
Knights have three states : Guard, Attack and Move. Guard makes them stand still, blocking Goblins, Attack makes them track down nearby Goblins and attack them while Move makes them move towards the mouse. The states can be changed via a popup GUI
Goblins will travel in groups and generally cause havoc. I have yet to add different classes of Goblin but this is in the works